That the Reach is supposed to be (and is) filled with Bretons and has been fought for between Bretons and Nords for centuries, yet the one and only pass between High Rock and Skyrim is in Solitude and the area is almost inaccessible from High Rock due to literally unscalable mountain ranges on the west side? I honestly tried to get my character up to the edge of the worldmap and couldn't find an area where it was possible. How exactly did they get there to fight over it, and why are there many more Bretons in Markarth than Solitude?

The Forsworn own the REACH that is why. They are not exactly Bretons but they were chosen as the race to represent them. Forsworn are more like Breton/Nords from what I can tell.

There are still loads of Bretons there (Look up demographics for skyrim on UESP), and lore says the territory has historically been fought over between the 2 races for centuries.

By the sea probably and there must be an entrance somewhere along the border. I've already found two entrances, or exits: one that leads to Morrowind, south-east of Riften and another one that leads to Cyrodiil, south-east of Helgen. They look like wooden gates. The road from Solitude crosses the border and leads to Jehanna; the path should be there, at the end of the road to the west.

http://www.imperial-library.info/sites/default/files/gallery_files/cyrodiillargelowrescr7.jpg

About Bretons in Markarth; people travel and Markarth is not that far away from the border.

The actual in game map is supposed to be slightly differing to the one of the lore, or so I am told. It probably makes better gameplay, but not being 100% true to lore.

I would assume the lack of a road or pass is exactly why they weren't subsumed by High rock or Hammerfell, and were able to keep a unique identity and religion, at least until the Nords decided they had aspirations above their station.
Is it not a fair assumption that some of those listed as Bretons are Reachmen whose ancestors were more 'Imperialised'? Those who may have been tempted from the wilds by the lure of civilisation?

I'm sure they could climb over or something.

wow i never relised how big of a world it was

This.

Besides that:
- The Reach as a modern day region goes up as far north as Deep Folk Crossing, which is on a mountain pass that could easily be crossed and which leads directly to High Rock.
- Current day regional borders in Skyrim aren't necessarily the same as they were before.
- Bretons are used to living in quite inaccessible mountainous regions. Much of High Rock is exactly that: inaccessible and mountainous.