Plumbing – Are all plastic compression fittings interchangeable

Find a better store, if the ones you are going to don't have 2" PVC in full sticks (or possibly look higher up in the store you are in, and get someone to drag it down.)

In MY house, the only place you'll find 1-1/2 or 1-1/4" is the tailpieces on the sinks - as soon as it hits pipe, it goes up to 2", because it's not that much more expensive and it greatly reduces the odds of having any issue with it. I don't recall having any trouble finding it in full sticks. The main line is 4" (not 3") for the same reason. Builders looking to squeeze maximum profit don't do that sort of thing, but builders who plan to live in the building for a while sometimes do.

In retrofitting, you might need to use 1-1/2" due to space constraints. In any case, if you are redoing the pipes, you should probably bring the venting up to current code while you are at it.

Yeah, a handy DIY person could do the re-routing of those hot and cold water line connections. You would have to deal with shutting off the water supply, cutting out all the old stuff and then re-plumbing in with new piping and fittings. Hopefully you get all the parts that you would need in one initial trip to the big box store. Do be aware as you get ready to re-route the pipes as close to the wall and as low as possible you may be faced with that electrical box back there being in the way.

After all that the quote really does not sound so bad considering the nature of today's prices for things. But it is totally doable if you have the gumption and determination to see it through. It may even be the occasion to acquire some new tools!!