Greg's thoughts on a couple things for your perusal;
Your idea of using pastels with a clear coat over the top: I recall doing this back in my weathering train car days. The airbrush blew the pastels off, was really frustrating. It can be done, but it's a goofy process I'd try to avoid.
Your asking about clear coat, specifically Future: I've had nothing but trouble trying to spray future (it dips nicely, and I agree with Cadet Chuck, I hear it works good on floors). I'd rather see you use any of the clear coats mentioned already, that's just me.
But then I'm all the time reading of folks says "Huh? I spray it all the time, no problems". So who knows.
Oil washes: glad you have some expert advice from other loyal followers, cuz I know nothing.
And no worries re the Hedison quote. BTW, his name (David Hedison) came up at Eurchre last night. Hmmmmmm, odd.