RAW, yes.
Nothing about the ability says you can't attack through your echo, and earlier, in the 'Manifest Echo' ability, it explicitly says you can attack through the echo. Being blinded and deafened would grant you disadvantage on that attack, but as you 'can see through the echo's eyes' or w/e, then that more specific text negates the blinded condition (for the specific scenario of seeing through the echo, but not for other scenarios like trying to fistfight a gnome with your actual body while using the manifest echo ability).
Twitter, no.
"The intent is that you can't do anything with the Echo Knight's echo while using Echo Avatar, other than scout with it. The text stating that intent is mysteriously missing from the book. We'll fix it in a future printing! Sorry about the confusion." link
Twitter is not rules text, and repeatedly has had things that are incorrect, not included in future printings, not reproduced to the actual errata website, etc.
Should it be allowed?
Depends on your table. At 7th level, Wizards, Druids, Clerics, Sorcerers and Warlocks are beginning to get some serious long-range attack capacity. However whether they use flock of familiars to electrocute people at long range, flying mounts and fireballs to carpet bomb armies or 300' eldritch spears to snipe people through windows or what have you, will depend heavily on group. Many groups simply don't do such tactics, or think of them, or always tend to start combat within 5' of enemies. Likewise other tactics casters use that are very strong depend often on the individual caster's knowledge of D&D mechanics.
Echoes being 1000' range assassin ghosts is pretty cool and honestly a perfectly balanced and thematic ability if casters are using some of the better spells on their lists (the large aoe perma-stunlock spell keeps shutting down encounters in my wednesdays game with a single cast), and it's fairly easy for enemies to counter - the echo is not hard to destroy and it will take a while for another one to amble over, which will also potentially give away the echo knight (echo assassin?)'s position, at which point the assassination gambit is up.
If the party has mages using Firebolt and the occasional Clairvoyance as their major magics, clerics that are purely healbot-mode, and enemies (this is the big one) who see a ghost walking up to them and go 'huh I not understand' like enemies in a CRPG where you've cheesed the AI somehow to win uncontested, then it will be too strong.
However, all kinds of games exist, so it's impossible to say whether it's too strong, fine, or too weak as it could be any of the three depending on table.
But Long Range Assassination Is Unbeatable
Perhaps. But an Echo Knight is not actually very good at long range assassination. A ghostly figure moving 30' towards a goblin encampment per turn that cannot Hide without a houserule is just not a very good assassin. It gets spotted automatically by passive perception, unless the DM very generously rules that spotting a magical image that cannot hide is Moderately difficult or higher. A goblin sentry with a shortbow in a tree who spots this non-hiding creature (passive perception says: yes) can defeat it on average (assuming a 7th level Echo Knight) with a roll of 12 - they are very likely to achieve this on their first or second attempt, and the range of a shortbow is 80/320. This is a CR 1/4 creature. The Echo Knight is level 7.
Now if the enemy is asleep and hasn't set any kind of watch, or relies purely on sound to keep watch (such as a permanently blind creature like a Grimlock - although, they have Blindsight - they can in fact see even totally silent magical illusions, RAW) they might let the Echo get into melee range, at which point the Echo Knight unleashes his attacks, maybe he even action surges and unleashes incarnation and etc. He might outright kill something with low hp. But that's a very cherry picked situation that favours the Echo almost entirely. It is far more likely that it is popped before it gets into melee range, or gets to attack once but only wounds rather than kills the enemy.. at which point they lash out and destroy it, and are on guard from that point forward.
If sent multiple times, it becomes increasingly likely a half decent scout could figure out where it is being sent from. The range is 1000 feet, and that is not that far, in terms of perception distance and travel time. On a battlemap it's a long way, but in asymmetric assassination, it's about 2 minutes' walk. You can't meaningfully 'circle around' without heavy cover to disguise direction and it will become increasingly clear where the image is coming from with each one that is destroyed.
Compare that to something like Summon Shadowspawn which can travel from up to 600x80' (~48000') away before engaging in combat, can Hide as a bonus action (meaning it doesn't automatically get spotted at max range by a sentry, at bare minimum), has an aoe fear scream, and 35+15/lvl hp (for reference, that's 7 goblin shortbow arrows as a third level spell), and what we see is that the long range attack echo is not actually best in class for long range assassination. There are actually better options than summon shadowspawn, but it's a simple thing that requires you to cast 1 spell and then tell it 'go kill those goblins over that hill' with no real thought or planning.
Upon the Echo's demise, enemies would become aware of an enemy and have time to prepare, set further sentries, hunt down the echo knight's party, or flee.
If you just look at resource cost (1 action) and damage per round (the echo knight's full dpr) and cost of it failing (nothing), it seems amazing. That will only work in a game with a DM who has very static, almost computer-game enemies. Even animal-intellect enemies will move away from a dangerous area. They are actually kinda famous for that, having multiple dens, running away if there's suspicious behaviour nearby etc.
Effectively this just gives a Fighter one of the many options Wizards already have. Just one. They still can't teleport, go invisible, save or die people, find the location of enemies/macguffins, steal thoughts from people's minds etc. If this was level 1, when wizards are down in the trenches with everyone else tossing out cantrips, then that might be a bit op given they also get the good armour and a strong attack. But at level 7? Eh.
