Is this homebrew Sorcerer of Whispers sub-class balanced?

Question

The Sorcerer subclass below is a home-brew sub-class for a campaign setting that's not related to traditional settings such as Forgotten Realms. The subclass is intentionally supposed to be good at aspects of spy-craft and information gathering--whispers. They may not be as good at other things, and that's okay as long as they aren't underpowered across the board or overpowered.

I'm concerned primarily about balance, but also about other things I might have missed, like if there's some clear reason that a feature of this class would be very hard for the DM to keep track of.

A few notes and preemptive comments:

• Concentration. The class has one feature that changes how concentration works in a very restricted case; I know this is controversial, but I disagree that all possible changes to concentration are necessarily unbalanced (for example: Arcane Abeyance and Artificer's Homunculus Servant do much more powerful things with concentration already; Matt Mercer's home-brew Cleric Moon Domain has a more flexible concentration feature than the one here). If there's a specific balance problem with the concentration feature in this class, however, I would love to know.
• Air. I know the concept of "Air" in this class is not identical to the traditional 5e concept of "Air". This sub-class is for a non-traditional setting.
• Domain Spells. A common problem people cite for Sorcerers is a lack of domain spells. I've added domain spells to this class, like with the Aberrant Mind sub-class. However, as with clerics, I don't allow Whisperers to swap these domain spells out at higher levels. This class is supposed to be good at specific things, not at divination and transmutation (or whatever) generally. Players are of course not restricted in terms of the other Sorcerer spells they take.
• Homebrew Spell. There is one home-brew spell included in this class's domain list, Whisper Sense. It has a duration of 10 minutes, and many of the class's features require that the whisperer be concentrating on it.

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Whisperer of Word and Air

The Whisperer of Word and Air sorcerer is attuned to the primal concept of Air. They gain powers over sound, light, communication, and the air itself.

The primal concept of Air is associated not only with the mundane air. Rather, it encapsulates the space between entities in the universe and thus is largely concerned with the transfer of information and with perception. Whisperers are often sought out to provide assistance with communication and information gathering as opposed to help with the local weather. Whisperers often have a knack for picking up rumors and stray information, and stories tell of Whisperers who knew everything about a town after spending a night there. In myth, the Whisperers of Word and Air tend toward solitary and remote contemplation, but, being Whisperers, no mere distance from others could make them truly isolated.

Child of the Breeze

1st-level Whisperer feature

You gain proficiency in the Perception skill. Additionally, you no longer need to breathe, and you can speak without breathing or moving your mouth.

You automatically learn the spells in the Air Spells list, below, at the associated levels in this class. These spells count as Sorcerer spells for you, but they do not count against your number of spells known.

Air Spells

• Level 1. Whisper Sense, Sanctuary, Message (cantrip)
• Level 3. Darkness, Silence
• Level 5. Call Lightning, Sending
• Level 7. Freedom of Movement, Greater Invisibility
• Level 9. Commune with Nature, Telekinesis

Voice of the Wind

When you cast the Message cantrip or the Sending spell while concentrating on the Whisper Sense spell, you may choose any number of locations within the area-of-effect of the Whisper Sense spell to also receive the communication. Any creatures at these locations receive the identical message or sending and may respond according to the respective spell's normal rules. You may choose whether the responses of all targets of the spell are carried back only to you or whether they are carried to all other recipients as well. Locations that are underwater or that do not contain air cannot be targeted.

Closeness of Air

6th-level Whisperer feature

You have developed an attunement to the concept of Air, and have learned to send your magic along its whispering currents. While you are concentrating on the Whisper Sense spell you gain blindsight with a radius of 30 feet, and, when you cast the Sanctuary, Freedom of Movement, or Greater Invisibility spells, you may target any creature in the area of effect of the Whisper Sense spell as long as you know the creature's location. When targeting a creature in this way, you do not need to be able to see the creature, to have a direct line-of-sight to it, or to be within the spell's normal range of it.

Whispering Currents

You may cast the Sending spell without spending a spell slot or providing components by instead spending 2 sorcery points.

Agent of Diffraction

14th-level Whisperer feature

When you cast either the Darkness or the Silence spell, the spell does not require concentration as long as its center is located inside of the area-of-effect of a Whisper Sense spell on which you are concentrating. When cast in this way, the spell's radius is 20 feet, and the spell ends when your concentration ends. For that duration, you may not use this feature again, but you may use a bonus action either to move the spell to another valid location within the area-of-effect of the Whisper Sense spell or to toggle the spell's effect from silence to darkness or from darkness to silence.

Charmer of Winds

18th-level Whisperer feature

You are so attuned to the concept of Air that you can traverse it with ease. You may spend an action and 5 sorcery points to grant a flying speed of 60 feet to a number of willing creatures including yourself no greater than your Charisma modifier (minimum of 1). The creatures must each be inside the area-of-effect of a Whisper Sense spell on which you are concentrating, and the flying speed of each creature ends when either the Whisper Sense spell ends or that creature is no longer within the spell's area-of-effect.

Answer 1

This is not balanced

Comparing this to the Sorcerous Origin: Draconic Bloodline, using the values from Detect Balance, which assigns a point value to various features and is pretty reliable on those it covers.

Draconic Bloodline - Resistance to common damage type (4), Extra Hit per Level (5), Natrual Armor AC 13 (3), Expertise on a rare subset of a skill (1), one Language is likely not worth a full point. Total 13 points.

Whisperer: Proficiency in a skill (2, although the strongest, so maybe 3), no need to breathe (4), and a cantrip (2). Domain spells do not include cantrips so the message cantrip seems out of place there and is included here. The other effects are not available with a point value, but they seem pretty narrow let say (1). Total 10 points: not quite as good as Draconic Bloodline but not unbalanced.

However, the subclass as written also has domain spells.

The small number of spells known is one of the defining restrictions on the Sorcerer class. This feature doubles the number of spells the sorcerer knows at each of the listed levels (for example, at level 3 normal sorcerer knows 4 spells, one with this feature knows 8). It changes how Sorcerer plays and materially weakens one of the main limitiations. Clerics know all spells on their lists, so the domain spells do not give them this huge boost in how many spells they know.

Adding this feature on top of the others is not balanced by any conceivable means. Compare this for example to the "Divine Magic" feature of the Divine Soul, which gives you one single additional spell known, not ten. The Aberrant Mind Sorcerer has Domain spells, and they essentially are the entire level one feature, except for limited telepathic contact.

If you opt to retain this feature, you need to cull nearly all the others.

I don't think it even makes sense to reveiw the rest after looking at level one, but below are a few additional observations.

Level 6

Other Sorcerer subclasses typically get one feature at 6th, not two. Drop one.

Closeness of Air

This has game mechanics issues. Whisper Sense is a Concentration spell, and so is Greater Invisibility. As soon as you cast the latter, Whisper Sense ends. So this will not work as intended. And mucking with how concentration works fundamentally is not worth it.

Your are overloading the Whisper Sense spell by tacking other features onto it or making them depend on it. If you want the class have features, let them stand on their own, not depend on a spell. You might have a feature that modifies it in a simple way, like Teleknetic feat does with Mage Hand, but if you continue to change and modify and morph what it does with multiple features, there is likely a better way to express that.

If you want the spell to see invisible creatures, then it should do so out to the range that matches spell's effect area.

Seeing invisible in itself is pretty strong. It likely would be a powerful feature all by itself for the level and your theme when always on at 15 feet. Compare for example the Divination Wizard, who gets seeing invisible creatures within 10 feet it as one of the options for Third Eye on level 10 (there is some others flexibility, but it is the strongest, my wizard has it on all the time).

Level 14 and 18

The issue with bolting everything onto the Whisper Sense spell continues. Get rid of that. It's like your whole subclass is a one-trick pony that consists of casting that spell all the time to do the cool stuff you want it to be able to do. Just let them do it, and limit duration, range and number of uses.

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P.S. You could include the text of Whisper Sense here if it is your homebrew. D&D Beyond is unlikely to go away, but it would be easier for readers of the class. The spell itself seems OK.