forget to mention one of the “turn arounds” I found, is to make a “Master Mesh” that has all material slots assigned, then I can duplicate it several times, then re-import from other mesh that has the same materials but not all of them.
pros:
it worked exactly as it should and it’s way easier, do it once and for all!
cons:
well, I guess you already know, so many un-used material slots!!! that needs to be removed!!! one by one! so it defeats the whole purpose!