Yes, by quite a bit
Note: personally, I love the idea behind your homebrew and would love to see it in a game, but here is my objective analysis
To determine if a class archetype is balanced, you should compare it to other archetypes. I'll break your subclass down by level.
At level 2, your character would effectively gain the benefit of a level 3 warlock feature -AND- a limited version of a permanent guidance spell.
While the limited version of guidance is relatively equivalent to the Savant features you see in other classes (provides some substantive, but setting-specific benefit), the Fiendish Familiar feature is a little above curve. Having access to an enhanced familiar takes that specialty away from the warlock (not to mention you get it a level earlier). Compare this feature to Improved Minor Illusion (since both provide a new spell with primarily out-of-combat applications).
When you choose this school at 2nd level, you learn the Minor Illusion cantrip...
When you cast Minor Illusion, you can create both a sound and an image with a single casting of the spell.
This is a very small benefit while your enhanced familiars have many applications (they can manipulate objects, fly, scout, and have their unique qualities listed below which have some combat applications or strengthen other applications)
- Lemure: Devil's Sight
- Dretch: Fetid Cloud
- Imp: Invisibility
- Quasit: Scare
The strength of Tradition Spells largely depends on how easy it is to find spellbooks and spell scrolls in your world, so I'll just assume it is okay (if access to spells to copy is rare, this feature becomes very strong).
Unrestricted Conjuration is inordinately powerful as the demon summoning spells are able to get away with a lower level spell for the same power as a result of this downside. Compare summon lesser demons to conjure minor elementals. They summon the same strength of creatures with a spell slot of one level lower. This feature is the most unbalanced in your subclass.
Deceptive Summons doesn't make that much of a difference thanks to Unrestricted Conjuration as the fiend summoned by summon greater demon isn't hostile anyway, but without this feature it scales to a 5th level spell conjure elemental similar to what Unrestricted Conjuration does. The only other spell this affects is gate, but if you've already summoned the demon, you don't really need to go find it in the Abyss later. (Related on what knowing a demon's name actually does)
You are comparing these features to something like Potent Cantrip...
Starting at 6th level, your damaging cantrips affect even creatures that avoid the brunt of the effect. When a creature succeeds on a saving throw against your cantrip, the creature takes half the cantrip’s damage (if any) but suffers no additional effect from the cantrip.
...which is clearly a much more menial effect to strengthening spells.
This feature has a virtual equivalent in the Split Enchantment feature of the School of Enchantment:
Starting at 10th level, when you cast an enchantment spell of 1st level or higher that targets only one creature, you can have it target a second creature.
However, the second creature still gets a saving throw, while this feature just gives you another summon. As such, it is too powerful. Without the benefits from level 6 though, it would be less of a problem since the demon's summoned would get a save in the same way as the enchantment school's feature.
Safe Summoning is another way to remove the downside of the demon summoning spells which comes across the same problems as Unrestricted Conjuration, but at level 14 this is less substantial. You are effectively reducing the 1d6 rounds (3.5 on average) to 0 rounds by using your reaction.
[from summon greater demon:] If you stop concentrating on the spell before it reaches its full duration, an uncontrolled demon doesn't disappear for 1d6 rounds if it still has hit points.
Wizards have quite a few strong uses of the reaction (one is the shield spell), so this is likely not especially strong and is comparable to the War Magic features Arcane Deflection...
When you ... fail a saving throw, you can use your reaction to gain a ... +4 bonus to that saving throw.
...and Durable Magic which makes it harder to lose your concentration (both of which are lower level features):
... While you maintain concentration on a spell, you have a +2 bonus to AC and all saving throws.
This additional feature for Signature Spells is strictly better (albeit by a small amount) than any other school as those schools don't get a second capstone option. This would be fine if the subclass as a whole were slightly weaker, but as I've discussed thus far, that is not the case. The archetype can already choose summon lesser demons as a Signature Spell which is already very strong with the buffs the other features of the subclass gives to that spell.
However, one level 4 spell is often worse than two level 3 spells, so this is only unbalanced as it gives a bit of flexibility (which is not that big of a deal in the grand scheme of things). I would almost always just use summon lesser demons and another level 3 spell instead of summon greater demon.
Level 2 features outweigh the benefits of other subclasses.
Level 6 features make demon conjuring spells much stronger and hugely unbalanced by spell level.
Level 10 feature is only a problem because of the level 6 features.
Level 14 feature is fine.
Level 20 feature is strictly better than other subclasses and therefore unbalanced (but not by that much).