Yes, you roll for each breach location. The design takes into account that most players will expand their game with add-on packs, so there might be different sizes availlable or, once a collection is large enough, there will be slight chance of running out of models. :)
No, the +1 tile effect is not cumulative. I am sorry for the misconception, "In case you don’t have... ...is not cumulative." should be one paragraph to make it clear.
P.S. you can use email@example.com for rules querries as well.
For The Last Stand mode, when in the spawning phase, are rolls made for each breach location, even if there are no alien tokens to bring in? If so, are the +1 MV bonuses from each location (if rolled) cumulative?