Whew, a lot of good questions. Let me try and share my thoughts and try and answer them for you.
First, the direct drum is a common failure point on these transmissions. The fluid pressures to engage and maintain engagement come through the case rear output shaft bore, through the output shaft and out through the part of the shaft that the direct drum sits on. A couple sets of sealing rings are involved. The steel sealing rings in the rear case bore, and teflon sealing rings on the stub end of the output shaft.
This problem is why I did 2 things in my rebuild. First I re-sleeved the rear bore to steel, and second, I added one piece teflon seals on the stub end where it rides in the direct drum.
Your situation as I understand it exhibits a broken steel ring. Could this be the source of the burned up direct drum? Absolutely. I suppose the better Q is how and why did it get broken. If you read the diary on this point in reassembly, you will see I commented that installation of the output shaft was an exercise in being gentle. *I* broke a steel ring! They are like glass. How any why it broke would be a mystery I'd be curious to know the answer to. That rear case bore is a critical item, how does it look? Any damage? If so that case needs to be sleeved to be reused.
Next the output shaft SHOULD use solid teflon seals on that stub end.
Can you reuse the direct drum? possibly. IF the bore is clean and undamaged, and IF the steels will ride up and down in the drum cleanly and easily, it can probably be reused, but I cannot say definitely without seeing it.
Sun gear and reverse drum dog ears engagement. FORD says wear limit is 1/16 inch. Re-Use? You make the call. The sun shell is reasonably cheap to replace.
I think your case end clearance will not be altered so much you cannot reuse the same end thrust washer (yellow) but again, that's my best guess.
If I were you, here is what I would do. 1) Carefully and honestly evaluate the rear bore for damage. If you find some, have it sleeved.
2). Put solid teflon seals on the output shaft and size them.
3. Evaluate the direct drum. New steels and plates no matter what.
4. see if the reverse drum and sunshell are all that bad or over 1/16 inch (pretty rough measurement using your eyeballs.
5. Treat yourself to a $15.99 HF Digital caliper.
If you need help on installing the one piece seals and sizing them to a new or reused direct drum let me know. I can offer some assistance, but you will have to ship it to me. and pay return shipping.
I think you can rebuild this trannie with bubble gum and baling wire. It is a strong tranny and is not needing of as much attention as the A4LD or 4R/5R series. BUT, that direct drum needs special care and feeding.
Hope this answers your Qs.
ps. depending on age and mileage, you might want to consider either a) replacing the EPC solenoid, or at least b) checking direct drum operating pressure at the direct tap.