Don't have a huge knowledge about the subject, but I do know that I've some times seen booking class A a few weeks out, I don't know if this classifies as dissapearing quick.
For 3: It depends on the fare price of A -> C and A->B->C, as the latter will probably be a bit more expensive you'll have to pay the difference. I have a booking CPH-BCN-ARN-CPH, if I attempt to change it to CPH-BCN-CPH it says I'm entitled to a refund of about 15€ (haven't attempted to change it) but changing it the other way would only cost the ~15€