The Echo Knight requires a lot of work.
I've played an Echo Knight, and then later DM'd for an Echo Knight. My experience with the subclass has been that the DM is going to be making a lot of rulings. As a player, my DM and I had to start making a list of interaction rulings for the various features. Knowing this, I did the same thing when I DM'd for an Echo Knight. So the short answer here that doesn't get into the calculus of the rules is:
The DM and player should discuss and agree upon a ruling and then just be consistent moving forward.
How I ruled it.
I'm not going to hem and haw my way through the technicalities of trying to interpret this question from a "rules as written" perspective, because honestly, the rules as written here are woefully unclear. Instead, I'll just tell you how I played the class, and how I ruled it later as a DM.
To me, the ruling that seems the most probable as the intent of the feature is that when using Echo Avatar, these two things are true:
- For the purposes of things you do in your corporeal body, you are blinded and deafened.
- For the purposes of things you do as the echo avatar, you can see and hear as though you were in the echo's space.
With this scheme, if you make an attack with your corporeal body, for the purposes of resolving the attack, you are blinded, which means you have disadvantage on the attack roll. Ruling it this way is for me motivated by something from my days as an amusement park manager. I used to sit in the office and watch security cameras, one of which was aimed at myself. I would try to throw balls of paper into the trash can or at my coworkers by judging my target's position on the camera screen, rather than actually looking at it. As you probably guessed, I was much less accurate this way than if I was actually looking at it, and that is how I look at trying to attack corporeally while looking through the echo's eyes. To me, the primary advantage of this ruling is that it is simple and easy to apply. In my recollection, there were no real ambiguities to resolve for this ruling's interaction with other combat rules. You know how the rules work when a creature is blinded and deafened, and you know how the rules work for a creature that isn't, and that is all you need to know to make rulings about this feature when you go with the ruling I outlined above. This ruling did not create the need for further rulings. Sometimes when you come up with a ruling, further play leads to needing more rulings as your ruling interacts with more features. That didn't happen here.
It should be noted, however, this is still better than being blind without the echo avatar. The rules for "Unseen Attackers and Targets" state:
When you attack a target that you can't see, you have disadvantage on the attack roll. This is true whether you're guessing the target's location or you're targeting a creature you can hear but not see. If the target isn't in the location you targeted, you automatically miss, but the DM typically just says that the attack missed, not whether you guessed the target's location correctly.
If you are just blind and deafened, you are guessing where your target is. With an echo avatar surveying the field while you are swinging your sword blindly, you don't have to guess your targets location. "I can see that there is a skeleton standing right in front of me! I will swing my sword in that direction."