You should not do that with or without earplugs, especially not now, but you already know that.
You will have to do a lot of internet research before you can choose what you can do in the gym, but not now, and until you understand how all movements affect inner ear you should not do anything.
What do C1 and C2 have to do with the feeling of liquid moving deep inside your ear when you bend over and lift a bike? Or with any dizziness in the elevators? Of course it's more likely to be inner/middle ear.
If they are saying or implying a clear CT means inner ear membrane hasn't been injured, they are already saying enough for you to know they don't have much experience with inner ear membrane injuries.
So they are saying tinnitus from years ago is causing you tinnitus now? OK. They did not know what caused it then and they don't know what's causing it now.
Tinnitus, dizziness, and other symptoms you have mentioned are symptoms.
You should do your best to try to let your injury heal and to understand your limits by researching on the internet, not by trial and error. And you should not push to check how far you can go. Sure, avoid salt and take a mild diuretic, but no point lowering your ICP with diuretics if you're going to pump it back up and increase it with running around or lifting. Lower ICP means less explosive pressure on inner ear (via cochlear aqueduct). Avoiding loud noise etc means less implosive pressure (via middle ear). You need to avoid both, at least for some time, at least for a couple of months, especially until you understand all about how pressure affects inner ear, in order not to make it worse if you get better.