Quote from bthec18 >>

I’ve always interpreted the attack from the echo’s position as the echo making the attack because it made more sense than you teleporting back and forth, especially when you account for extra attack, action surge, and getting multiple echoes later on 

 Very similar to what Davyd said, but basically the way Dunamis/Dunamancy works in Critical Role is it is all about "potential". Basically the echo is an aspect of your character from a different timeline that, in this case, made the decision to be on Point B instead of Point A (it's wibbley wobbly timey wimey stuff, so don't think too hard about it).

Your Echo IS YOU (but with different AC and can maybe hover depending on how you read the "On your turn, you can mentally command the echo to move up to 30 feet in any direction (no action required)."

So any weapon masteries you have, your echo has, because it's just you from a very slightly split timeline where you made a different decision on positioning (basically). You aren't teleporting, you're just attacking through the space you would have been in if you did something different.

I don't know if that helps, but that's how I see it as part of the Critical Role lore.