I just had a gentleman bring a boat to us that his "friend " gave to him for free. The fabric was real bad on the surface. I recognized this star inflatable design as a late 80's version, and I was surprised that it had made it to the point where the material outlasted the glue. We were discussing the same issues then we unrolled it, and every seam I could lift up with a thumb nail, and it was not even hot from the sun yet, or inflated. The idea that you have repaired a seam already leaves cause for concern. You will probably (from my experience) have to rework seams on a regular basis. This becomes a problem when on the river on a multi day trip. keeping an old glued pvc boat alive can be a huge problem. It is the stress of inflation that will open up a tube not the air pressure. so if you do repairs try to go 1 1/2 inches over the effected area with a quality 2 part adheasive. and if possible use a light colored fabric. My personal recommendation is to not waste your time. I have never seen a customer with that kind of fortitude and determination.
To summarize: Glued PVC boats generally have the glue go bad before the fabric goes bad. I have seen some of our own stuff that is 15 years old that we could not separate with a heat gun, but heat and humidity are the real culprits for glue deterioration over time. Glued PVC boats in Alaska seem to have no problem what so ever. A glued PVC boat in a locker on the beach in Puerto Vallarta will not last a year. We tried it. The locker was right next to the beach, and in the sun. The very worst possible conditions.
For more information about boat repairs including hypalon urethane and neopreme, see this web site.