I think you have a compression valve. Is that valve on copper pipe? Not sure why the leak. If that is copper pipe, I think the more likely problem is that whoever installed the valve did not use enough torque. Also, check that the valve is fully operable, and rotation is not impeded by the toilet.
If I am correct in all those things, the remedy would be to pull the toilet for access. Then loosen the nut and pull the valve, giving access to the ferrule. Lube the ferrule and threads with pipe dope or other suitable lube. Then tighten the valve back in place with two fairly long wrenches. This is not the job for stubby wrenches or weak hands.
It seems unlikely that the problem was that the pipe was too short. If that were the case, he could have used a shallower escutcheon, or even omitted the escutcheon letting you put on a thin split escutcheon later.
If the valve is not readily operable, a different approach is needed. I am not a plumber.
That leak problem is not the toilet. As to what he should have done, if copper pipe, I think he could have made the valve more accessible. See
https://terrylove.com/forums/index.php?threads/shut-off-valve-is-blocking-new-toilet-installation.75038/ #3