Steve, as far as I know there's been no controlled studies to answer your questions. I'm no bowyer, so take this for what it's worth.
How the string grooves are cut makes a huge difference. Cut at a 90 degree angle, the string may shear down each side. That is the problem with a lot of older bows.
Limb material and age probably will be a factor, especially the glue that was used, but that's just a guess.
Padded loops add surface area. May or may not be a big help, but it won't hurt and it makes for a quieter shot with recurves.
Tips can be replaced and string grooves modified. This may make the bow ok, may not.
I know there are people who get away with shooting high performance materials on old bows. There are lots of variables to consider here. The condition of the bow, draw length, draw weight, arrow weight, etc. etc. etc.
FWIW, I use HMPE materials on my self bows. One has antler tips, one has horn, and one has osage overlays. All have been shot a lot. So far, so good, except the antler tends to pop off (glue problem, not a string problem).
One thing is for sure. If the bow fails and has a HMPE string on it, nearly everyone is going to blame the string.
Personally, I'd stick with B-55 on bows not rated for HMPE materials.
Chad