On a pass or revenue ticket, if the person was willing, they would first have to get authorization from the captain, but as long as there was no one else with a higher priority already trying to get the jumpseat, that would be fine. Example: sometimes an NRPS might offer to take the jumpseat on a full flight so another non-rev can get on (who is not qualified to sit on the jumpseat, like a parent or spouse of an employee.)

As far as being on a revenue ticket and taking the jumpseat instead, I doubt you would see that happen much! I know if I bought a seat, I'm going somewhere as just me- not playing flight attendant this trip, and just want to relax and be a "real" pax for a change! LOL Theoretically it could happen, though.

Jumpseating is a whole separate issue from sitting in a cabin seat, and handled differently.

As far as jumpseating on other airlines like the pilots do, there would have to be an interline agreement set up between the two airlines. Some airlines have reciprocal cabin jumpseat agreements, some don't.

Hope that makes sense.