Are the alternates part of your team? Will a Wolverine Fear Itself with a Wolverine AvX(125pts) revert alternate satisfy the requirement of having atleast one character from the AvX starter set?

What scenario are you referring to? The suggested one for the AvX events? If so, this would be a question for your local judge, as he has the final say for that event.

I would say no, but I am not your judge.

Ask them.
I am unaware of any official rule regarding whether a sideline figure meets any sort of requirement for floor or the AvX suggested requirements.

To me, it's about that figure starting on the team.
For a floor event (and this is like a floor event to me, only instead of a point requirement it's a figure requirement of 1 or more), it wouldn't matter if on the first turn you merged your entire floor into a figure that didn't meet the floor requirement. For AvX you could merge your Thor, Iron Man, and/or Cap into Avengers Prime and I would be okay with that.

You started out with a legal team on the map.

To have them start on the sideline, though, they may never see play.