(1) My understanding is you cannot cover the box as you've done since it contains live connections. If the wires inside the covered box were totally retired, you may have a different situation. You would need someone here to give a more informed opinion of the code.

(2) Why not just remove the double gang box, do your patchwork, and install a single box. Keep box fill requirements in mind.

1) I actually just got done exposing that one box I had hidden, and installed a blank plate on it.

2) I would remove that double box and replace it, but literally all the wires that are running into it take up all the extra room. I wouldnt be able to fit it all into a single gang box. I thought I read somewhere as long as the wire are accessible without having to remove any permanent part of the wall, than the box can be covered. Well, it is accessible, only partially covered over.

You can find sectional cover plates at L* or HD that you can configure with a switch and blank, or visit an electrical supplier to special order a plate with a blank and switch opening.

Another post was talking about a mud ring to use a double box for fill and bring the dimensions at the opening to one cover. Probably have to recess the existing box slightly to accomodate the mud ring.

A mud ring will only work if using a single gang mud ring to replace a double. If the box is a 2 gang rather than 4 square a mud ring won't fit. An electrical supply house should have either a 2 gang cover plate with a single switch opening (got one from mine last week) or a filler that blanks off the second switch hole in a standard 2 switch plate. The big box stores stock the sectional plates as normel pointed out but they look terrible

what about just putting in a dummy switch?

For 39 cents that is the easiest and cheapest solution.