I’ve seen a case where valsalva maneuver has resulted in retinal detachment, and that is a well-known enough outcome that it musn’t be dramatically uncommon. You hear of cases here and there where it does result in eardrum rupture, and I imagine you’d be at increased risk of this if you had a history of perforation resulting in a weak patch in your ear drum. So you do have to take care and do it gently. Generally fluid in the middle ear just needs time to clear out. Sometimes a few months, unfortunately. Yawning, chewing, pushing fluids, getting enough sleep for general illness recovery are all good.
Peroxide does work to debraid wax from the wall of the ear canal, and it is the main ingredient in a lot of wax removal drops. Most people tolerate it. It will not have any effect on middle ear fluid. If you have an unknown perforation in the TM, peroxide will hurt like hell in the middle ear.
I actually have voluntary control over popping open my eustachian tube (if it’s not too gummed up), and tensing that muscle does cause a bit of a sensation in my jaw similar to what onegooddonebad describes. It’s useful for rapid airplane descents. A good minority of people have that muscle control, and another group has control over the tensor tympani muscle (makes a rumbling sound in your ears when you tense it). I hypothesize that there is also an overlap in control over these muscles and the ability to wiggle your ears.