It's not terribly difficult to put up new drywall (drywall = sheetrock, same thing). You'll have to get out what's left first, hard to tell what kind of job that will be. Then you tape the seams and put a little mud on them, and you're good to go for new tile. Tile does need a smooth surface, though, and is pretty unforgiving, so I think you're pretty much stuck doing the new sheetrock yourself or having someone else do it -- I don't think you can patch that many holes and then put the weight of the tile on top of them without risking the patches falling out.

Only you can answer that question. Is it going to bug you for years if you don't fix it? Some people just like to know that things are perfect, regardless of whether the flaws are visible or not. My husband would have to fix the wall, I would be quite content to leave as is.

There shouldn't be grout between the counter and bs because grout doesn't give. When transitioning from your horizontal counter to your vertical bs, you use caulk, only. So now someone, your GC is fine, has to remove the grout in that line, probably with a multimaster, and replace it with caulk, being mindful of the type of caulk with respect to your chosen materials.

Do you mean the size of the subways that you will use to replace the granite are not the same size as the granite back splash? If so, you could lay them the other direction; up and down, as opposed to whatever pattern you have now. One row like that will look OK. How about a close up picture of what you have now? I don't care for the look of the white subway tiles with your granite but I don't live in your house so my opinion on that really doesn't matter. A subway that blends with your granite would look very nice and you wouldn't have to take the granite off. Either way, your wall behind either/both will be damaged.