I have the following Situation:
3. Yes, you can use separate cards for fill + key. But there is the possibility that the synchronization of the key and fill signal gets lost. So this way may work well or may not, ...
I use two "Decklink Mini Monitor" for key and fill ans I know what happens, when the signals become unsynchronous. It looks like a shadow at some edges between fill and background. The key-mask is at an other position as the fill. So there were small areas (depending on the speed of movement), where fill is cut away and areas with no fill - so black.
When the movement stops or you use still graphics, there is no visible negative effect.
In the next days, I'll try a "DeckLink 4K Pro", I bought some months ago. It is installed in a PC far away from my ATEM-mixer and so I wasn't able to test earlier.... Maybe the new and cheeper "DeckLink Duo 2" could also do synchronous key/fill (maybe someone can confirm the synchronous work of both cards)