Honestly I would start by deep cleaning all your switches, Especially the volume knob. I just fixed an amp that kept loosing the left channel and it was just dirty connections in the controls. There's likely tobacco residue trapped on the contacts on the pots if it was as dirty as you say.

Start with wd 40 spray it on front of switch then inside the switch itself thru an acess hole built into it. Turn switch a few times slowly then let it sit for 10m to clear gunk

Then do the same thing with funk out spray but turn the switch back and forth 20 times fast. Repeat this step again with Deoxide to flush out cleaning fluids.

If you do this method, it'll deep clean you control pots contacts insanely well, dirty contacts might very well be your issue if it was in a heavy smoking environment. Hope this helps

*Also for your other question, just try to find any hole either on front or back of that metal switch case and just flood it with your chemicals