I dont understand how a plastic shot cup over a fiber wad would melt. It is ahead of the gas. Only if the fiber wad is not sealing properly and gas is escaping could I appreciate how the shot cup would melt
As for the pattern when using plastic wads Ive found they hold a tighter pattern
I don't understand it either, and I came to much the same conclusions that you did, but even lubed fiber wads would not solve the problem. Makes no sense to me.
In contrast, I use Low Density PolyEthylene (aka LDPE) wads, 0.06" thick, directly on top of >80 grs of 1.5 or 2 fg Swiss powder in my rifles and never had any plastic melting in my bores. Recovered wads look as new. The rifle wads and the plastic shotcups seem to be of very similar, if not identical, plastics, but I can't testify to the chemical nature of them.
In the past, I have used thick artist watercolor paper as a shot wrapper as well as the mylar wrappers sold by most reloading suppliers. The paper might slightly tighten the group (presumably less shot deformation on the barrel walls). But it is a PITA to use it and takes up valuable case space.