#1 - not likely.

#2 - you should be able to get the seat, unless the fare rules of a r/t H fare prevent you from booking the return you want. For example, if the r/t H fare requires a minimum stay that you won't meet then you can't change your ticket to get it. Basically it comes down to both finding availability AND meeting the fare rules. If you can do both then you should be able to just pay the change fee. If not, no love for you.