Law is a profession that is like medicine in some ways: you typically don't want a proctologist doing your brain surgery (unless you're a liberal, anyway)
Estate planning lawyers are the ones who would do this. There is no such thing as a "gun trust" either. Any legally established trust is sufficient to purchase NFA items. Some trusts have extra wording to deal with firearms, but this is not necessary.
Except they don't need to be 21.Works just like buying a handgun, you must be 18 to possess, but 21 to buy from an FFL. Beneficiary only has to be 18.It is not necessary, but it is a good idea, since there are legal limits on who can take possession of the items (for example, if you have your child as the beneficiary, you need wording on how to handle it if they are not 21 or over when they inherit)
Yeah, if I was really worried, I'd send a money order. Of course, with a trust, you can e-file and use a credit card.