I saw a video where ATT simcard was used on verizon iphone 5 and it worked perfectly. No reason why you cannot use the straightalk sim on Verizon iphone 5. But to cut the micro sim to nano sim you will have to be careful. I read someone had already accomplished that without any cutter or anything on hofo.
But on the side note Sprints Iphone 4s was unlocked for i guess 2 months before they pushed an update for locking it. Not sure if verizon is gonna do that.
As for the activation part you should be able to use the phone even after activation if its unlocked. You already have a verizon iphone 5 or are you planning to get one? If you already do have it just pop a sim and check it.