can't even turn the handle to turn on or off, I guess it's been this way since original construction in 2007 and the valve has never been exercised.

What kind of plumber would put on a valve knowing you can't even turn the handle?

The supply piping is CPVC, what is the best way to fix this? I am not even sure a quarter turn will work here.

Should I just cement a 90 elbow then a short piece to point up but tilted to the right get the pipe away from the cabinet wall and use a straight shutoff valve?

If you do not want to disturb the CPVC, you could take the handle + stem off and drill a hole in the cabinet wall (about 1 1/2 in dia) to enable turning of the handle. I expect the stem is seized by now, so replace the stem while you are in there.
From what is visible in the pics, that wall does not look like an outside wall.
The brass coloured handle is a nice touch, don't come across it very often.



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i would just take the handle off and turn the stem with a plyers.

Sum, either a valve like pictured or a brasscraft with the triangular handle.




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LOL packy
Me too… but seriously if you have to change to valve anyway, just 90 it over away from the side of the cabinet

I wouldn't blame the plumber, looks like a new vanity was installed either over from where the original was or a smaller cabinet was installed where a larger one was. Either way, I would take off the handle and try turning the valve stem with pliers, if that doesn't work and the valve needs replacing, elbow it 90* so the handle has clearance.