CDG to Montparnass

If your flight is on time with 8:15 arrival, then four hours to clear immigration and get to Gare Montparnasse shouldn't be a problem, especially if you don't have checked bags to collect. The risk arises with the fact that transatlantic flights are sometimes delayed by hours. Personally I wouldn't buy a nonrefundable train ticket within about 12 hours of my flight's scheduled arrival time. Have you explored whether there's a refundable or exchangeable option?

PARIS MONTPARNASSE 1 ET 2 refers to the 2 of the 3 Halls at Montparnasse. This refers to the main station and you will just have to get to the station and check the train board to see which track your train is departing from. Could be from either of these 2 Halls. BTW Hall 3 is also called Montparnasse Vaugirard, located next to the main station. From CDG, take the RER B to Denfert-Rochereau then switch to the M-6 to Gare Montparnasse. Or you could take a taxi.

Or you coukd take a bus:

https://www.oasbus.com/cdg-paris-montparnasse/

There is no mention of that bus service on the official airport website, is it running yet? There used to be "le bus direct" before Covid but that was suspended. In any case, at 17€/ticket it was especially poor value unless one was travelling alone.
In my view, the best options are either a taxi (most comfortable, 58€ flat fare) or RER B + line 6 metro from Denfert-Rochereau station (beware, there are many stairs).

To my knowledge, the new OAS service operates as scheduled, but I have yet to see one of the buses.

If I were attempting to make a connection at Gare Montparnasse from CDG during rush hour in the morning, I would seriously consider the RER B/Métro 6 connection.

The timing would have me using the RER/metro as well but you do have to be able to easily handle your luggage on lots of steps -- RER stations have elevators/escalators but the metro often requires lots of long steep stairways up and down.

There are really many steps at Denfert-Rochereau going between RER B and line 6, it can get nasty with heavy stuff. I did it almost every weekend with cabin-sized luggage for 2 years and hated it.
Rush hour traffic is bad around Paris, but if you land at 8.15 you are unlikely to be in a cab until after 9, and the worst of the Paris-bound traffic should be behind you.

LOL we actually made that transfer about 15 years ago when we were much younger and stronger. I carried our messenger bags and my husband behind me was carrying our suitcases. I had our passports, money, cards etc in my moneybelt under my clothes and lucky thing as my husband was surrounded by three good size male pickpockets and restrained and frisked. I have never seen this kind of open assault/pickpocketing before or since. Usually skilled pickpockets operate with finesse; these guys were just thugs. An older woman ran up and screamed at these guys in French and they ran off -- because he didn't have a wallet in a pocket, we didn't lose anything. But yeah lots of stairs in that transfer and for us again at Edgar Quinet which was our stop.

Are people sure the OAS buses are actually operating? I ask because a friend of mine came across the site and wanted to try them for his upcoming trip but when he tried to buy a ticket this week no matter how many times he clicked on purchase nothing happened. He has decided to take a taxi but if people know the service is actually running, maybe he can buy a ticket at a counter at CDG.

The OAS bus is not listed on either the official airport site,

https://www.parisaeroport.fr/en/passengers/access/paris-charles-de-gaulle/public-transport

or the easyCDG site,
https://easycdg.com/bus-cdg-de-gaulle-airport/

I thought those buses had been closed a couple of years ago.

The Bus Direct (formerly Cars Air France) did shut down completely/forever sometime the summer of 2020.

This Bus OAS looks to be a company that is trying to recreate that route — but from what JHK said, it doesn’t sound like a viable option.

Janet that is horrible what happened to you all a few years ago !!! Thank goodness you were well prepared but still that is awful.

yeah it was a bit of a shock. Something that blatant is very rare. Most pickpockets are skilled professionals and you don't know you have been hit till you discover your stuff is gone. I think these were just garden variety jerks who saw a guy with his hands full and decided on a quick score. I am a fast walker and had gotten out ahead a bit so he was alone on that staircase when I turned and saw what was going down.