Who is going to find them out and hold them to account? No one.
What do you mean? Everyone! Europe would not be happy with that. The USA would not. Russia has every incentive to expose such an action if it took place. Belarus would also be very incentivized. Seriously, think about it.
This is a war and both sides are to blame.
No, this is an invasion. Ukraine did not attack Russia, even Russia acknowledges that. You can say an invasion is justified and make that case but that's the position you're starting from, Ukraine was not looking to war with Russia, you can't just hand wave a "uhh, both sides" here and that does all your cognitive work.
Calling it ethnic is one of the justifications Russia gives but if you think about that's not something we would use to justify an invasion from anyone. It's bullshit, this is political and a power grab. Putin isn't exactly secret about things.