I think you guys are looking into lore way too much. I agree sakurai likes to portray characters as they are in their games if he can help it, but the fact is that shadow CAN and HAS done everything sonic does here in smash.
If "this doesnt represent how they could actually fight/this represents what they are known for" was as important as you guys seem to think, none of the echoes we have now would be echoes.
Dark Pit is known for his use of the staff and other weapons he uses in uprising. I cant even remember seeing him use silver bow prominently at all in the game. But here he is, an echo, because he is perfectly capable of copying pit DESPITE the fact that he does not use the same weapons as pit. Completely disregarding his potential and representation outside of taunts etc.
Everyone swore up and down daisy would focus on athleticism and flower power, but here we are with a peach echo. This happened because it's possible (I guess) to do everything peach can do. Once again, despite her potential to do all these things and represent her uniqueness, we got an echo tha can work.
Lucina, for whatever reason, uses a two handed sword in the same style of (one handed style) marth. There have been plenty of posts at this point that explain how absurd it is that she could even copy marths fighting style based on the lore of her game. Again, here we are with a character that does what another character does because it works.
At this point you have to see a trend. Echoes dont have to portray a character down to the tee in its moveset. So long as the character's personality is prevalent in its taunts, victory animations, voice lines, and minute differences represent the character they are fair game to be an echo.
I think boxing yourself in to only seeing or accepting things a certain way is the quickest way to disappoint yourself. Do I want shadow to be an echo? No, I would prefer a semiclone that doesnt revolve around spamming spindash and spring in neutral, not to mention he's been shown doing all kinds of cool and unique things. However, I do see that if push comes to shove, yes shadow can be an echo and it would make 10x more sense than the other echoes we have now. To simply denounce the possibility because you want something more, is foolish. It's okay to prefer something, just don't forget it can easily (and is more likely) to go the other way.