Can we get choices that really matter and have a effect?

[Edax]

These two are their replacements that fill the exact same role in the story.

https://masseffect.fandom.com/wiki/Padok_Wiks

https://masseffect.fandom.com/wiki/Urdnot_Wreav

No, if you shoot Mordin or Wrex in Mass Effect 3, their replacements will not appear in the game. Their characters and their respective roles are dead by the point.