1. Compulsory movement action - can you choose not to move and just surrender this action?
2. You decide to activate your Wizard and 3 figures within 3" - do you have to declare this before taking any actions? Does movement have to be the first action for all of the figures?
3. If you take somebody out in melee, do you get to move back 1" just as you could if you had beaten them and they had not died?
1. Yes.
2. There are two types of activation in this case:
a) you decide to activate upto 3 soldiers with the wizard in the wizard's phase - you can activate these models one-by-one, each model takes both it's actions but does NOT have to make it's movement action first
b) you decide to activate the wizard and upto 3 soldiers as a
Group Activation - in this case each model must take one action then each can take it's second action in any order you choose. This first activation HAS to be movement action first.
3. RAW I'd say yes - the push back rule says "once a winner has been determined", and a model that kills it's opponent has obviously "won", and the model can't push it's opponent back nor stay in combat (as they're not there) but it there is nothing to forbid it stepping back itself.
However, if the model was out-numbered in combat and defeated one opponent note it can't step out of combat with the other enemy (in the FAQ).