Its a no brainer for them to get involved, same with T-Mobile.
They have skipped the "4G" (as america call it) wireless speed completely to jump to LTE, Its their big selling point compared to rival companies that they can offer these LTE devices all of which are Android phones, because the iPhone 5 is "only" capable of 24Mbs through the standard 4g network.
If Apple get all the big Android phones banned, then they lose out big style.
I don't think they can, is it something like a general patent so anyone can buy it and samsung has no choice who can purchase the license. I'm sure I read that in one of the articles.
Indeed, its a industry standard thing. Although Samsung refuse to see their patents as being industry standard so feel within their rights to refuse Apple access without Apple paying a stupid fee (iirc, they wanted something like 6 times the fee others pay).
Best outcome for Apple will be they win outright and Samsung devices end up banned.
Likely outcome is that some of Apple's points are upheld and either they: a) won't have to pay for the patents at all, because Intel already do and Apple get the chips from Intel.
or
b) They will have to pay for the patents under FRAND terms, costing peanuts.