As you say, when paste softens up filler you have a major problem, because sods law, the failure is not at all easy to sort out (even with a magic wand!) I guess good old molten hoofs and bones aka size is a step in the right direction, makes more sense than diluted paste or PVA.
I was guided a slightly different direction after college and for the longest time I have defaulted to Beeline Primer Sealer for ensuring minimal fun and games with dodgy surfaces.
So this one product binds down any dusting surfaces, evens out the porosity of a variable surface (filler, plaster and whatever else might be there), stops the paste penetrating, and provides an even paste-friendly surface too. (Should be sealer-primer by rights, no?)
Sounds too clever for its own good, but it works, dilutes 3 to 1 for most situations, and is one of those bomb-proof products that gets a knowing nod from the few who use it.
And I am confident it isn’t a snake oil product, because when I came back from US a few years ago, I couldn’t find it initially, (didnt look too hard, I know) and took my chances on a job and sized the surface with diluted paste. Wrong, how many lifting edges in the lining paper? I soon went looking very hard, and quickly too, and started again, Beelined the surfaces and never looked back.
Gardz is the American version with a couple more qualities, but for problem solving surface prep for wallpaper only, I favour Beeline as we referred to it - before Beeline did everything known to man.