Before I even get too deep into properly reading your responses to the individual points. just a couple quick skim responses:Pardon the huge multi-quote reply, I will try to be succinct
You use the same strategy as when buying tools, clothing, or a 40k army; if you can't pay for everything you want in one lump sum you break the purchases into useable chunks and start with what you're going to use most. When it comes to RPG miniatures, you start with the most common encounters and work from there. My collection started with undead and dungeon vermin.
I dislike the idea of a DM soliciting players for $$ to purchase group miniatures. I encourage players to buy their own PC and henchman miniatures but do not require it.
Reaper Bones, Nozlur’s Marvellous Miniatures and Pathfinder Deep Cuts are all budget miniature lines of decent quality. You could get set up with a gameable collection of undead, dungeon vermin, and bandits for under $50.
I am a "less is more" DM who sticks to curated bestiaries for whatever setting I am running and don't feel the need for every creature in the book to make an appearance in my games. Even then, my collection is pretty substantial. I don't think it is practical or even desirable to account for dozens of slightly different humanoid races and multiples of the 300+ creatures in the Monster Manual.
The "how many owlbears are needed" question feels like a non-issue, the answer being "however many you want." I imagine them as solitary apex predators that congregate briefly to mate so I own 1. If parliaments of owlbears rampaging through the forest are a thing in your game, I guess you'll need to buy more.
The sweet spot for most combat-heavy RPGs like D&D is warband/squad sized conflict so I buy as many miniatures as I think will be practically used, which is about a dozen at the top end for things that come in big numbers like giant rats, bandits, ape-men and zombies but more often less. In the rare event that I need to represent more creatures on the table than I have miniatures for, I fill out the ranks with Litko plywood bases of the appropriate size as stand-ins (which are the first to be removed as casualties, naturally).
With all due respect, I think it goes without saying that space is needed. Obviously, if no one in the group can secure enough space to comfortably host, gaming with miniatures is probably a bad idea.
I do not feel it is practical to accurately represent terrain and find that a measure of abstraction is perfectly fine, especially if PCs are blowing through multiple biomes in a session like a video game. I have a Paizo battle mat that's dungeon flagstone on one side, generic green grass on the other that works for the vast majority of encounters. I own number of themed grids like desert, city, and even a ship but they are rarely used.
Again, I think less is more applies to terrain. I use a little 3d dungeon terrain, mostly doors, chests, altars and pillars. I also pack a small number of representative outdoor items for whatever biome they party is in (e.g. palm trees for tropical forest). I don't feel the need to cover the table in wargame terrain; a handful of strategically placed, inexpensive thematic terrain pieces can make a battlefield pop without wasting a lot of time.
That said, I am looking for good dungeon tiles to up my tabletop game. Dwarven Forge tiles are absolutely gorgeous but experience has shown me that walls get in the way and setup takes too much time.
As a general rule, I buy miniatures with utility in mind and all of them are going to get used multiple times.
Then again, the money shot of my current game is gonna to be a showdown with Juiblex and in this case I don't mind blowing 60 bucks on a miniature that may only be used once (it will probably be recycled as a super-shoggoth or something though).
1) You've already spent more time thinking about miniatures use in what you've typed than most game texts referring to them as part of the games assumed style of play.
2) In some of your responses, you've made what I'd call "design choices". Those choices, some undoubtedly seemingly obvious (I'm of the GM provides minis school of thought also) can and should then cause follow up questions to be asked. That can or should then lead to feedback in the rules themselves, IMO.