Ok Jack & Trevor,
I will give you my oppinion on your questions .

1: I would strip every panel down to bare steel & Spray a coat of PPG's DP or DPLF sealer down .
I say this because it sounds like your car has been painted a few times, as all GM cars were lacquer up till the mid 80's when GM started appling Base Coat / Clear Coat to some of their cars.

2: You CAN apply body filler to this DP sealer if sanded lightly for adhesion.( PPG's recommendation)

3: The less material you have on your car the longer the paint job will last if done correctly! Less is better today as these products are all high build .

4: I mention PPG products as I have used them for almost 30 years and know them inside out and have never had a problem.
So i reccomend PPG brand.

5: For your filler Primer after your body work is done I reccomend
NCP-271 CORROSION RESISTANT PRIMER. or K36.
If you use NCP 271 I forwarn you it stinks like all get out, but works excellent! (Make Sure YOU use a charcoal respirator sparying all these products!!!!!!!!)

6: I would use DCC 9000 Single stage with du-5 hardner for my top coat. You can also use the DP sealer for a sealer before top coating to seal off all primer before paint application. You do not have to apply a sealer over your 500 wet sanded primer if you dont want to, as long as you have no bare steel showing through primer! This will also keep some paint cost down, as these products are not cheap.

7: On the door jambs I would water sand the areas you can get to by hand easy with 500 grit, the areas that are irregular use a RED Scotch bright & a little soap in warm water and scrub as good as you can, as this will be alright as long as all the shiny is gone on the paint.

I hope this is some help & know you wont like the idea of taking all panels to bare steel , but you wont regret it in the overall finish
of your job when done . Plus it also is a safe bet that you wont have any paint problems arise because of something under all your hard work that isnt compatiable .

SprayTech