Here's my two cents on the Echo issue:
When looking at whether a character can or can't be an echo, the question shouldn't be 'the character could very well be unique, so they shouldn't be an Echo', it should be 'is what makes them unique makes them impossible to be an Echo'.
For the latter, remember the previous example of Dillon/Sonic that we talked about. Dillon does share some similarities with Sonic, enough that it's been brought up from time to time and he could theoretically reuse some assets from him, but what makes him unfeasible is that Dillion's proportions make him a poor fit for Sonic's moveset. Sonic's legs are long and he relies on a lot of kicks, Dillon's are stubby and he can't use those kicks without multiple tweaks, and the same goes for a couple other moves like the spikes. What makes Dillon unique is what makes him unfeasible for the role as he'd require multiple changes to work, something that goes against the idea of Echoes since by nature they're extras that don't require a lot of resources to function.
For the former, the fact of the matter is that when you get down to it, most clone characters in the game can easily be unique as all of them have lore, game play or traits that can easily translate to being a character that plays differently from their counterpart. Lucina for instance canonically doesn't fight the same and should be closer to Ike if anything due to inheriting her father's fighting style, Daisy never uses Toads, float, vegetable or other similar moves, but that doesn't prevent her from pulling them out for hers, even examples like Dark Pit can be different since he has his staff and can use the other weapons, but that didn't stop him from being a clone. What made those characters unique was not enough to prevent them from being made clones when push came to shove, and we can't ignore that just because it already happened for them, but didn't happen for the other characters yet.
The same can hold true for examples like Dark Samus. Yeah, she can fight differently from Samus, you can look at her boss fights for that, but nothing about that prevents her from being an Echo. Canonically she does use moves like the Morph Ball, canonically she does steal Samus' abilities, and canonically she does use Samus' form as a template for her own. The character by nature is a derivative of Samus, and while there is a lot you can do to make her unique (and I would love to see that personally), there's also a lot she shares that can make her an easy clone. She doesn't have what Dillon has that prevents him from being an Echo (ex. unfitting proportions), and thus she can easily have what happened to those like Lucina happen to her. That's not impossible and we have to keep that in mind when arguing her potential as a character, and the same holds true for characters like Impa or Hilda.
In short, when analyzing what can make a character an Echo, don't exclusively think 'well this character can be unique, so they can't.' Think about what makes them unique and what makes them similar, and then think about whether what makes them unique can prevent them from drawing heavily from what makes them similar to their counterpart.
*Also, fun fact: Dark Samus's animations for her AT reuses some from Samus. Her Phazon tentacles uses Samus' Down Tilt, the Phazon spheres uses her Up Smash, and the Scattershot uses her Missile animation. So that example is actually a mixed one since her similarities from Samus is what allowed her to use her counterpart as a base for her AT.