Can't answer for Ryanair, but in our case :
1) Schedualling initially assign a/c type for route, "theoretically
" taking into account a/c performance requirements (engine ratings, weight of a/c etc etc)
The pre-planning function takes care of what should happen in two to seven days time, taking into account a/c mx requirements etc.
Then ops "line-up" the a/c on a nightshift for the next day, taking into account all the above, plus last minute changes caused by a disruption / unserviceabillities etc etc etc.
2) Any route on the network taking into account the above parameters, or any unserviceabillities on a tactical basis (ie on the day).
For example, a inop APU means a/c can't fly to INV because they have no GPU support.
A typical day will see 'plan A' change on a regular basis to 'plan X' if it's a bad day.
3) Again, depends on carrier. We fly 10-12 sectors per a/c daily.
4) All contained within CAP371 & company FTLs. ( Keep away from this subject as it's a real bone of contention with flight-crew !!!
) Basically there are set legal limits.
In a nutshell, as long as the crew are within legal limits for the duty, then they are required to fly.
Hope that this all makes sense ??
Rgds.