site will probably be a good help to you. The wikipedia references
for the Arsacid empire will probably be helpful as well. I myself have found the references section to be immensely helpful at times. As they are legitimate sources.
scholar.google, of course will help you. (Please note, for me, as a person who follows links like a trail to gain information, google.scholar can be less of a help. But an excellent place for actual publications and so forth.)
1. I highly doubt it. There will definitely be very large differences. Almost every ancient language has evolved significantly up to modern times. However, as far as I know, there is very little actual documentation for that period of time. So you could probably get away with the modern language.
2. Chances are, he'd probably be enslaved. Slaves were a lot cheaper then actually paying for an interpreter. But keep in mind that in those times, slaves were generally treated very well by their masters, especially if they were high ranking ones. Frequently they could have been considered extended parts of the family.
3. Both were fine, from what I've found. In fact, they seem to have been fairly equal in terms of followers.
Note. Do some proper research into Mithraism and Zoroastrianism, particularly Mithraism. As it seems very much like one thing on the surface, but is nothing like it when you actually study it.
4. Don't know. I read at one point that they considered themselves to be friends of the Greeks. Which meant the real strife was with the Sassanids. Who considered them traitors as a result. Even the academia slagged them for it.
5. Yes. Definitely. But I don't know what system.
6. I don't know. I'd have to look a lot more extensively to find out.
Good luck. Sorry I can't help much, but I'd have to do a lot of research to do that, and I haven't the time.