Just a few questions:
1) Does Nemisis Bio-Nuclear Blast "ignore" Skrull/Shapechange/Super Senses?
2) Can a single figure receive multiple uses of the X-Men (or Teen Titans) TA? (could 3 differnt figs "take the damage click" in order to heal 1 X-Man 3 times)

This thread has a discussion about Bio Nuclear Blast. Right now it is not clear whether Super Senses would activate against it. Some people feel you are making a ranged combat attack so it would activate. Others think the "as if" language means you are not making an attack so it might not activate. So we're waiting for a final word on how this is suppose to work.

As for Shape Change (and Skrulls is Shape Change), it appears that Bio Nuclear Blast does not target so it would not activate.

That would be fine.

Could Leech use Charge granted from the Crimson Gem of Cyttorak? (or does his trait prevent that?

Well, my gut reaction was no, it wouldn't work.

However, after reading what you have linked, it specifically says 'other' characters which means than yes, he could use it.

Yep, Leech isn't an 'other character' to himself. If his power would say 'Any character' then he couldn't use it.

Would Tabitha Smith take Mystics damage from a a fig (friend OR foe)that was damaged by a time bomb? (and Nightshade would be in a similar situation as well...)

It's not an attack, so Mystics does not activate.

Thats what I thought. Thanks Quebby!