If Exp. Boomerang and Exp. Ultron were teamates and adjacent to each other, could Boomerang's attack be a 13 because of the wildcard team ability that Ultron has?

Yes indeed.

Oh, yes...the Exp Ultron - Sinister Syndicate combo....brutal....but effective! But even without Ultron, you've still got Bullseye with his 11 attack that he can share...one of the scariest sights I've ever seen is Bullseye sitting on the corner of the building on the 3-buildings-and-a-park map with his buddies Boomerang and Mysterio....
Ultimate Hawkeye went down in a hail of gunfire before he could even get them in range....

How long does copying a TA last? Can you copy the Crossgen/Mystics TA? (e.g. if your wildcard gets hit during your opponents turn, is the copied Crossgen/Mystics TA still in effect?) Or perhaps the Kingdom Come and Danger Girl TAs?

You can change your TA for each action taken during a round. You cannot use two different team abilities to perform the same action. IE, if you use Superman Enemy to Outwit, you cannot copy Batman Enemy to boost your attack value.

You can, however, use Batman for stealth against a figure at range that would try to attack you, and then Mystics if a close combat figure attacks you in the same round.

Oh right! I assumed the word "action" in the PAC referred to 'your' action only.... didn't realize it was for 'any action'... Thanks!