Which is actually not applicable. Because first you would either have to prove they joined for financial gain, or that they were there to overthrow the legitimate government.
Now, if there was an actual call out for pay for people to go to Ukraine to fight, then it might apply. But there is nothing under that that covers volunteers.
And the same with the government. Now when the President of Haiti was assassinated last year, those were mercenaries that fall under that as they were fighting to overthrow the government of the country they were in. However, fighting for Ukraine in Ukraine does not apply.
No, they are not. Citizenship is not required. And in fact, when they were captured they were actually in a military uniform. Which is far more than we ever saw from the Jihadis in the Middle East and elsewhere in the last 2 decades. Under both Hague and Geneva, they are not mercenaries and are entitled to POW status.
And I have absolutely no idea what the DPR is.