Questions on covering

My Decathlon is in annual right now, and needs some repairs to the original
dope-on-Ceconite finish.
Our new A&P doesn't have much experience with fabric work, so he sought
advice from others. I'm feeling uneasy about the answers he got. A couple
of questions for those with paint and fabric experience:

1. Wag Aero sent us Poly-tak cement and poly-fiber tape to make the
patches, and poly-fiber products to paint over it. Why would they do that?
Will this work? I understood that mixing systems was bad, and voided the
STC.

2. The belly fabric is soaked through from an old oil leak. I mean really
soaked through. The dope is sloughing off the bottom, and 2 inspection
rings have fallen off. I feel we should recover the fuselage, but my
partner wouldn't go for it. We're going to try cleaning the belly out by
washing the inside with mineral spirits, stripping the outside with MEK, and
then repainting with poly-fiber. Does this have a chance in hell of
working?

3. Assuming this doesn't work, our next step is to try reskinning the belly
only. Looking at the fuselage, it looks like it was covered using the
envelope method. Just a guess, but it doesn't look like it's fastened to
the lower longerons with anything but fabric tension. When we cut the back
half of the belly skin off, what is to stop the fabric on the sides of the
aircraft from pulling up towards the top? Any advice on how to make this
repair work?

4. While stripping a few areas with MEK, I ran across some small areas of
paint repairs that were obviously not done with dope. The paint coat was
shiny and thin. Instead of dissolving in the MEK, these paint areas
disintegrated into little bits of crumbling film. Excuse the gross analogy,
but the paint behaved not unlike the way your skin peels after a bad
sunburn. What kind of paint could this be? Is it enamel or urethane?

Thanks,
Ed