I'm not sure what you mean, "presuppose its glitches, not OpenMW's glitches". Bugs in the game data is bugs in the game data, regardless of the engine it's on. Can you provide an example (real or hypothetical) of what you're talking about?
The one linked to here is a plain zip file with an esm. Very few mods are actually distributed as exe installers given the presence of mod managers.
- generally have MS Windows .EXE installers;
A mod can only be tested by people using it. Besides, the people who are behind MPP are well-versed in Morrowind modding, they aren't in the habit of releasing shoddy work.
- seem to be incompletely tested, totally untested, or known-problematic when it comes to, say, patching the bare original game in an MS Windows VM then importing it into OpenMW;
Huh? That's what modding is, modifying vanilla game data (in this case to change buggy data into less buggy data). They don't directly modify the original game files, they work like any other mod.
- when working, are going to directly modify the vanilla game data, which is antithetical to OpenMW's modding model
Any bug report should include the list of mods used. And if a mod or combination of mods causes a problem in OpenMW, so long as it works in vanilla and doesn't rely on external tools like MWSE or MGE, it likely is a bug that should be looked at.
- and will interfere with debugging OpenMW itself (i.e., one may submit bug reports others cannot reproduce, because the underlying game data is significantly different)
- and often depend on MWSE, MGXE, and other MS Windows "co-executables".
I have never seen an unofficial patch mod rely on a script or graphics extender. The one linked above certainly doesn't.