If it is found that the Russian-Separatist were in fact the ones that downed the plane how will they be held accountable?

They won’t be held accountable. Putin will give them a free pass.

A stern talking-about (since there is no “talking-to”) and sanctions from buying things from us that they have no intention of buying anyway.

Oh, and a strict rule about “No Anti-Aircraft Missiles” signs to be painted on all planes now.

None. These guys are accountable for nothing to no one. Welcome to the present and future of warfare.

Who is in a position to punish them? Putin alone. What are the chances that he will? You have better odds of winning the lottery.