Oh wow! That sounds rough.
I have a nice full arm cast from my elbow to my wrist (Oberarmschiene). I just learned that I will have it removed next week already, because they don't want to immobilize it for too long, but I was told it will hurt a lot after it's removed, so we'll see how that goes...
My thumb is indeed mobile, but my wrist is locked in place, so it is indeed quite uncomfortable to grip a mouse.
XCOM is a great idea. I played it years ago, but never made it to the end.