As far as i know, there was sonic, vicor pressing plant, hidcor, mareco, alpha and dyna (which also pressed wea's lp's) From my (user) experience, dyna pressed its own so the quality was quite uniform, except in instances where they would use a foreign master, as was the case with a lot of polydor, emi and warner (with dyna/wea had the license to sell here) and it's quite easy to spot a locally cut dyna pressing and one wherein the foreign master was used, you can see it in the dead wax. This would more or less answer question #4, wherein some dyna pressings (in the dead wax) would have their imprint and some foreign artists' albums would have been mastered using orginal metal parts. For example, i could say with certainty that ABBA's "Arrival", "The Eagles "Hotel California" and Chicago's "Gold Edition" (or is it Gold Record) were pressed using the foreign masters. As far as the PARI seal is concerned, the deterioration of local pressing quality had more to do with the period these records were released; and not the "seal". I bought a lot of albums that had the pari seal that and locally cut were pretty good like Frida's 1984 "Shine" lp (locally cut, dyna/polydor) or 10cc's "Windows in the Jungle" (pressed using polygram metal parts). Then again, dyna's locally cut "vitamin z" was crappy. Octoarts pressings was sad to say consistently spotty and used different pressing plants like mareco, sonic and presswell EVEN IF the company used original columbia masters, for example: Billy Joel's Greatest Hits. Orignal masters nga, pangit naman ang vinyl. Or the octoarts edition of "The Best of Earth Wind and Fire". Noisy due to poor quality vinyl. By the late eighties, everyone was already starting to produce shitty lp's. One possible reason why the Jo Stafford local version rivalled the original has nothing to do with the pari seal. It's most likely because the company which issued the lp used foreign masters (as in yung stamper galing sa mother company). From my own experience, the early mareco pressings were pretty good and the later ones, especially towards the late 80's was crappy, example: Rick Astley's first lp.
Nowadays, when i have no choice but buy a locally pressed vinyl (didn't have the money to all those that i wanted when i was a kid haha), i'd go by the period in which these were released; not for its pari or non pari status. For some reason, i wasn't able to buy Madonna's "Like A Prayer" lp; after looking at a friend's well cared for local album, which used original metal parts tapos dmm pa(!) the surface itself looked like it was pressed on crappy vinyl, so i don't want to buy that album's local pressing. By that time, dyna's pressings were no longer as good as it used to be.
Anyway, just my 10 cents worth, based on my experience as a collector