Trouble turning off supply valve - need it shut off but won’t fully close
Getting it to shut off completely is unnecessary. There are plenty of SE questions about swapping a valve or trying to get it to close more if you really want to. You're talking about a down time of minutes, not days or weeks; that's when I would suggest that, or taking it off and capping it.


Turn the valve off as much as possible. 
Flush the toilet. 
Take the supply line lose from the toilet and let it, and the toilet, drain into a bucket. 
Have towels down on the floor because it's going to get a little messy as you install the new equipment as fast as possible and hook everything back up.
Don't be in such a rush that you cross-thread or over tighten. A little water never hurt anything.


If you're really worried get a tiny brass cap to put on the valve while you work. It's probably 3/8ths compression thread and usually found in the isle with faucet repair parts - alternatively, get a 3/8ths compression thread plug to put into the supply line. Both usually require a few wraps of PTFE tape to actually seal - but again we're talking minutes, so whatever: towels, a bucket, and get 'er done.