If say the Pope was going to a Mass, does he have to be the main celebrant? Could he be a concelebrant (i.e. letting a Priest or other Bishop celebrate)? Could he simply attend and sit in the pews with everyone else and receive Communion like everyone else? Could he (or any other ordained cleric) act as an altar server, lector, etc.? Could a Bishop allow a Priest to be celebrant and the Bishop either be concelebrant or whatnot?
Just wondering.
As for the title of the thread, the simple answer is no. In general, the senior priest will preside, and most of the time “should”–especially with regard to the bishop. There are times when this might not necessarily happen, though. The bishop who ordained me liked to attend the first Mass for every newly-ordained priest (as much as he was able), and he sat in a special chair in the sanctuary in cassock and surplice to allow the new priest to truly preside at his own first Mass.
As for a cleric (bishop or priest) simply attending and sitting in the pews, this has a more ambiguous answer. Priests especially have an obligation and a right to concelebrate whenever they are at Mass. Deacons, likewise, but this isn’t always practical. In other words, there is more of an obligation to make accomodations for priests than for deacons. This may happen at, let’s just say for example, the Chrism Mass, where a sufficient number of deacons are already ministering, but (in a large diocese) there might be 100 deacons who are seated in the pews (I say ‘might’ because it’s still more proper for all the deacons to be vested). If there isn’t room in the sanctuary, priests can concelebrate from the pews (obvious example, the Papal Masses in the US in '08).
Priests are not outright forbidden to not-celebrate, but this is frowned upon, and there is an understanding that there has to be a good reason for this. If a priest is ill, or he’s travelling and not known to the pastor of the place, or if he simply arrives late, are some possible reasons. Some religious orders have their own rules on this: there’s at least one recent thread about it.
And yes, a priest could serve the Mass–either in cassock and surplice or even fully vested, but doing the parts which the altar boys normally do (if vested, he’s actually concelebrating). A priest can’t actually “be” a Lector, but a concelebrating priest could certainly do the readings for that particular Mass. Same for the deacons parts. The priest can’t “be” a deacon, but he could do the deacons parts, in the absence of a deacon.
The Extraordinary Form is also an exception, because priests can indeed “be” the deacon or subdeacon (and vest as such).