

Doesn’t hurt to ask. Not related to anything electrical, but “a while” ago my office chair mechanism started to make really annoying noise, metal grinding on metal screech. It was at least 6 years old at that point and isn’t high-end model by any stretch. I sent email to manufacturer if they could point me to a retailer who sells spare parts but instead they just shipped me a new mechanism for free, no questions asked.
Obviously if the model isn’t available anymore it might be a different story, but if it’s still on the market they might give a surprise.

Yes, but there’s a spesific order which you need to touch the wires so that it registers movement in one way or the other. You could also solder wires to another rotary encoder and use that (assuming of course that the new part electrically matches the old part).
Spraying contact cleaner in the case might work too while rotating the knob. Just drowning it in something most likely won’t help much, but if you can get the encoder off the board then you can sink it in a isopropyl alcohol and rotate the knob around. Just let it dry properly before putting it back together, contacts may spark when used and if there’s alcohol vapours around that might cause some new and exciting problems.