Mont St-Michel to Paris

Rome2rio isn't very reliable and might have messed up your results due to time difference.
There are direct buses from Mont St Michel to Rennes, several times a day. Then you take the TGV to Paris. This is typically the fastest option to Paris. I believe the bus company is Transdev.
In season (should apply if going soon), there is also a bus + train connection via Pontorson in the late afternoon, with a direct Pontorson-Paris train. This combo is sold by SNCF.

As for the lack of weekend trains from Caen to Paris: that is due to engineering works. It should be temporary.

The only scenario in which “one has to leave the tour group and find their way back to Paris” would be if you or your travel partner is sick. If this is the case, I would hope you would quarantine where you are until you are well. Leaving the tour does not necessarily mean going home immediately. During this quarantine time, you will be able to devise a plan to get to Paris and then home.

@ balso
Thanks for your suggestions; we are essentially referring to the similar thing.

"bus + train connection via Pontorson in the late afternoon"

I think connecting at Pontorson is not a convenient option because as I mentioned in my OP the bus (I believe only one daily, unless I'm missing something here) arrives at Pontorson around 6 PM and the last direct train to Paris depart at 6:14 PM. This is why one likely will have to stay overnight in Pontorson for the next morning train.

I used Tranzer mobile app (which Transdev works with) and found two options: A & B.

Option A:

Transits at Saint Malo & Rennes. I couldn't find a direct bus (you mentioned) from Mont Saint-Michel to Rennes; all the tools (Transdev, Rome2Rio and Google Map) required taking a bus to Saint Malo and a TER train to Rennes.

Le Mont-Saint-Michel > Saint Malo > Rennes > Paris Montparnasse

• BUS (BlaBlaBus) 2:55 PM —> 3:35 PM
• TER (SNCF) 4:29 PM —> 5:20 PM
• TGV (SNCF) 6:35 PM —> 8:03 PM

Option B:

Transit at Caen. It requires a long bus ride (2 hours) to Caen.

Le Mont-Saint-Michel > Caen > Paris Saint-Lazare

• BUS (FlixBus) 2:55 PM —> 4:50 PM
• TER (SNCF) 5:57 PM —> 8:02 PM

I likely will go with option A.

@travel4fun
There's no suggestion anywhere in the OP about skipping quarantine; the OP was asking how to get back to Paris from Mont Saint-Michel.
But thanks for the quarantine reminder, it goes without saying. Here is the CDC recommendation for quarantine and leaving isolation:
* Day 0-5: isolate
* Day 6+: can end isolation if symptoms improve
* Day 6-10: wear a mask unless 2 negative tests 48 hours apart

The Keolis bus goes from Mont St Michel to Rennes or St Malo. We took it from Rennes in June 2022 and were pleased. It dropped us off right at the Place Des Navettes and I think that's where it picks up passengers for return to Rennes or St Malo.

https://keolis-armor.com/en/5ey-Going-to-Mont-Saint-Michel.html

We didn't take the Pontorson train option partly because the short time between bus and train worried me, too. My impression is that the bus stop is very close to the train station and that bus is intended for people catching the train.

And there are multiple buses between Mont-St-Michel and Pontorson per day. There's a link to the bus schedules here, although it looks like they change after Sept 25 (probably seasonal).

https://www.ot-montsaintmichel.com/en/discover/visit-the-mont-saint-michel/access-the-mont-saint-michel/by-bus-and-coach/

From Arromanches there are a number of departures per day which are one bus to either Bayeux or Caen, then train direct to Paris. Use the website of the train operator SNCF Connect, rather than rome2rio (more often than not gives poor quality information) or google maps.
I know Rick recommends Rome2Rio- there is a large body of evidence that it just does not give correct information.
You have also been given good quality information now about Mont St Michel, again information directly from the relevant operators, which belies the third party planning tools.
I may not have done the journey but when I see that the bus operator says that Pontorson journeys run in connection with trains and that they terminate at Pontorson station I would be quite happy to book the through journey.
To me laying over at Pontorson seems way too cautious, almost making things more difficult than they need to be.

@PlannerMom:

Thank you so much for those links, it's exactly what I have been looking for. Very helpful!
So one can just take the Keolis bus goes from Mont Saint-Michel ( Le Verger) to Rennes (Gare Routière); and then TGV to Paris ... @balso also suggested bus+TGV but I wasn't sure what bus service to use. This is game changer as the total commute time is down to about 3.5 hours !!

Revised plan:
Le Mont-Saint-Michel > Rennes > Paris Montparnasse
BUS (Keolis) 10:00 AM —> 11:10 AM (next bus 5PM, 6PM)
TGV (INOUI 8710) 11:35 AM —> 1:16 PM (next 1:35 PM, 3:35 PM)

@isn31c:

Thanks for suggesting SNCF Connect for the Normandy itinerary; it really comes down to using the right website/tool. Now the trip from Normandy to Paris is very straight forward one:

Arromanches-Les-Bains > Bayeux - Gare SNCF > Paris Saint-Lazare
Bus (121) 1:47 PM —> 2:24 PM (earlier bus 9:25 AM)
TER (3314) 3:38 PM —> 6:01 PM (next 5:31 PM)

@ **balso, PlannerMom, isn31c**:

I have to agree with you guys that Rome2rio is not a very effective tool when it comes down to the nuts and bolts. This is why this forum is great, as people like you guys shared information that I might not have found online easily. So I'm very grateful for your helps; thanks again!

Now I'm ready for my trip :)

Ha, it was Keolis and not Transdev for the bus, my bad!
Regarding the bus to Pontorson: it is a shuttle bus timed by SNCF to connect with the train, so it is designed with that connection in mind. But going via Rennes is way quicker.