Not following what you are getting at there - the various different instances don't exclude each other if assumed there was a range of them in terms of power and not always all present at once. It is only his last note which makes them hard to reconcile - remove that and everything else falls into place with a fairly simple explanation consistent with other aspects of the lore i.e. that the Maiar were numerous.
Morgoth didn't need a production line to corrupt the Maiar who became Balrogs - he used influence to attract them to his cause corrupting them in the process - many of the changes they did to themselves - it wasn't like he did something specifically to each of them one by one.
It doesn't seem that all he attracted were corrupted "...of the Maiar many were drawn to [Melkor's] splendour in the days of his greatness, and remained in that allegiance down into his darkness; and others he corrupted afterwards to his service with lies and treacherous gifts. Dreadful among these spirits were the Valaraukar, the scourges of fire that in Middle-earth were called the Balrogs, demons of terror."
My personal feeling is Tolkien pretty much narrowed it down to 7, but almost sought to make that 3 later and discarded the rest.
I certainly don't get the impression there being vast numbers personally.
When it was talked about the balrogs they seem few in number, 7 to Ungoliant.
The only way I can reconcile it, because it is unreconcilable without some manipulation is that he at some points was referring to all the corrupted minions in effect as Balrogs, and somewhat later made Balrogs more specific. even thats a bit of a stretch to try to reconcile.
Its very hard to get away from "In the margin my father wrote: 'There should not be supposed more than say 3 or at most 7
ever existed.'" Key word highlighted. Maybe he just forgot or chose to revise what else he had written at that point.
It "hundreds", then "and there came Balrogs one thousand" then the balrogs (all of them totalling 7) to ungoliant, then "say 3 or at most ever 7". I dunno it just sounds a bit like he wasnt that worried about the exact number and definition and your just supposed to take them as being a strong force either in number, or in individual power and not get too bogged down in taking detailed notes and comparing works