Flight check in times within europe

Paris to Munich and Frankfurt to Paris are better done by train. In which case the answer is that arriving 15 minutes before departure is plenty of time.

As for the others, it depends a bit on when you're flying, if you have access to fast track security, planning to check in luggage and so on. But both Munich and Rome are large airports.

Hi and welcome to the forum. To answer your question, it is a matter of personal preference / caution level, whether you plan to check bags, and your airline. Some budget airlines close the check in desk 45 or 60 minutes before the flight. I view myself as moderately cautious, and for me, for flights within Europe, I would plan to arrive at the airport 2 hours before the flight, with the idea that if I'm a few minutes delayed, it probably would be fine. But I also take into consideration how I will be getting to the airport and what is my plan B in case of delay. Have a great trip!

90 minutes is generally sufficient, but I'm paranoid so I generally show up 2 hours early, and for CDG 2 hours makes a lot of sense. Ive been thirough them all and CDG is the only difficult one. Yes, they all have pre boarding for those that need it. No proof of need required.

Regarding the boarding for those with mobility concerns, you should probably arrange that with each airline beforehand.

Recommend checkin time of not less than ninety minutes. Checkin desks in some cases open 2 1/2 to 3 hours before a flight. Arriving three hours is not necessary. Often there is airline specific cutoff times for receiving checked bags or for checking in at the counter. Getting through security or the distance to the gate may need to be considered. Each airline has its on process for boarding. Some flights may require busing to the aircraft. In that instance I’ve never seen a separate bus for special requirements. Some airlines provide separate buses for business and first class passengers versus main cabin customers. Boarding processes for the many carriers that I’ve used in Europe, Asia and Africa tend to be extraordinarily efficient.

Mobility limitations have to be announced for the regarding passenger(s) in the booking process, minimum 48 hours prior to flight.
Based on the announced limitation class airlines arrange mobility aids and help at the airport.
Normally passengers with limitations board first and de-board last.

Also, do airlines have a priority boarding set up for those needing to
take extra time to board with mobility issues?

Just to board the flight early, you need nothing.

You wait for them to announce something like those with little kids and people who need more time boarding come to the front. I did this on a few trips when I messed up my hip. No one asked why on any of the flights.

To Badger's point, Paris to Munich can be done by train in 6 hours with a single train change, and tickets for 5 adults is in the 300 EUR range, city center to city center. There is no extra charge for luggage and seat reservations are included. By plane, leave Paris to the airport 3 hours before departure time, 1 1/2 hours in the air, 45 minutes to deplane and get luggage, 45 minutes to get to Munich city center. That comes to 6 hours city center to city center. I don't know what kind of deal you can get, but you will pay extra for luggage and seat selection. And you pay for transportation to/from the airports.

Frankfurt to Paris can be done by direct train in 4 hours, and again, tickets for 5 adults is in the 300 EUR range, city center to city center, no extras for luggage or seat reservations. By plane, leave Frankfurt for the airport 2 hours before flight time, 1 hour in the air, 45 minutes to deplane and get luggage, 45 minute to get into the city. That comes to 4 1/2 hours city center to city center, Again, pay for luggage and seat selection and transportation between the airports and city center.

FLIGHT Paris train station to Munich Train Station
To Airport from Paris Est 0:45, Early Arrival 2:00, CDG to MUC 01:30 minutes, Delays 0:30, Deplaning 0:30, MUC to Munich Train Station 0:45: Total time: 6:00

TRAIN Paris to Munich
Early arrival 0:30 (15 minutes if you are the sort of person that does not need to ask the sorts of questions that the OP is asking), Train 6:00: Total time: 6:30

Theoretically the flight is an hour faster, but let’s say it’s a wash, and it’s about what you think will be a easier journey. I would fly. I hate being stuck is a seat for 6 hours.

FLIGHT Frankfurt to Paris
To FRA from Frankfurt Central: 0:15, Early Arrival: 2:00, Flight: 1:30, Delays: 0:30, Deplaning: 0:30, GDG to Paris Est 0:45, Total time 5:30

TRAIN Frankfurt to Paris
Early arrival 0:30 (15 minutes if you are the sort of person that does not need to ask the sorts of questions that the OP is asking), Train 4:00, Total time: 4:30

Theoretically the train is an hour faster, but I still think it’s about what you think will be an easier journey. This one I would consider the train because my tolerance for a train ends at about 4 hours. Might come down to where my accommodations are located. Closer to the airport or the train station. Or flip a coin.

But neither is "better" for everyone.