Imagine your washroom having pipe leaking, and only able to get repair in the next 1 week (Say long holiday). You have off the main tap for 1 week.

To remove you have to hack it out, if you can hack it out, why not replace with a better looking piece?

All are complete conceal inside with the level for turning stick outside. Rubber can still be change. Maybe if you can post a picture is easier to see.

That because the initial developer/contractor may have over plaster. Or the valve was installed too deep. So to bypass, you still have to hack it out. After bypass you have to switch off main incoming if anything wrong in the bathroom / the area. And like I say if you can't get it repair fast, the main got to off until it repair.

If this is from your water tank, you need to check if you have main. Some house water tank don't have main (only in the roof), each bath room is independent. And If your valve connect to the water tank, that mean you can't cut the water until your tank run dry.

Than your valve could be for all the toilet. Older hosue have one main valve for water tank. If you need to service or fix leaking you got to drain all the water out before you can easly change it.

See the pipe size.

I am only a DIY, not expert in this area. Think a profession will know once when he check. Another method I can think of is go up the roof check the pipe at the same location where your valve are place below that way you can check the pipe size going downward.

Tank don't come with any valve. If the installer follow the common standard they will put it in between the tank and outflow.

No you can't. You need to buy adaptor to convert from 1/2 to 3/4 or vice versa. If from 3/4 to 1/2, you are reducing the size and water pressure will suffer.

Don't know what you want to do. But I am sure a plumber will be able advice you.