An outlet was discovered to be midway through a segment of unsafe (too thin gauge for circuit, no ground) wiring. Let's refer to point A as a junction to the rest of the circuit (which is all good to go), point B as this outlet, and point C as an important outlet at the end of this branch. There's nothing else on the branch between A & C.
Because the outlet at point B was hidden by furniture and not used, and there's already ~8 other outlets on this circuit (maybe more), it's been decided that this outlet could simply be removed. To most rapidly get rid of the unsafe wiring between point A & C, new wire was pulled through each end of the gang box and spliced, thus completing the circuit with new wiring. However, this leaves an unnecessary splice in a gang box at point B, and the box itself seems a little unsafe to me (it is a not very secure/stable, old work box).
To reduce electrical risks, it seems to me it's worth it (though not high priority) to re-run a length of wire from point A to C directly with no splice, and in the process remove the old work box. This is not that difficult and since splices and rickety junction boxes are some of the major electrical hazards as I understand it, it's worth upgrading: it will take 10 feet of 12/2 NM, pull part way through from point C to B and then pull the rest of the way through from B to A and get it all hooked up.
When all that is said and done, the rickety gang box will be empty and unneeded. Easy enough to remove if that's best, it's practically falling out of place now. What's the easiest way to plug the hole? Do I need to add drywall + joint compound + paint, or can I just put a gang box cover on the empty box? Guess I'm trying to confirm if it makes sense to rewire the splice out of existence, and if there's any problem putting a cover on an empty box or even somehow putting a cover on an otherwise open but discrete hole in the wall.