Paris city pass

do not buy a Paris Pass. If you think a Museum Pass would be of value buy it directly. The Museum Pass has no advantages unless you plan to spend your 4 days in museums in which case you might save money. Most museums require reservations and the louvre has limited slots for Museum Pass holders. I'd just buy tickets from the direct websites for those things you want to do. Eiffel Tower tickets are released 7 days ahead at 1 pm Paris time. (some are released earlier, but the biggest release is 7 days out.

Just walk up to one of the Seine Cruise docks when you want to go and buy a ticket.. There is a dock below Pont Neuf, one by the Eiffel Tower and one upriver from Pont d'Alma --

For transport, you do not want the Paris Visite which is the most expensive transport pass. Get a Navigo Easy and load it with 10 metro/bus tickets at a discount. If you run out, add a few more.

If you buy the Paris Pass, you will be over paying because about 30% of its price is commissions and profits for the promoters.
You can do much better than the overpriced Visite transit pass. You can buy the Museum Pass direct. You can just walk up to any of the several docks on the seine and purchase a cruise on the spot for the next boat.

Yes, avoid the Paris Pass. Simply book what you want to see, directly with the venue's own website.

The Musee d'Orsay does not require a reservation, but if you purchase tickets in advance, you can skip the ticket lines at the museum. The Louvre does require reservations but it is easily done on the Louvre website.

I also reserve restaurants. If you want to dine at Granite or les Ombres, make those reservations a few weeks in advance. Otherwise, reserving a day or two in advance should be sufficient. No need to reserve at cafés.

Your transportation options can be a bit confusing, if you would like suggestions, please state when you arrive (day of the week), and where you will be traveling by public transportation.

AND…did you know Viator is a 3rd party booker owned by Trip Advisor? They do not run any tours, just charge companies 20-25% to book for them. On any Viator page you can find the name of the actual company providing the tours and book directly through them.

For everything you’ve listed, as my forum friends have noted book directly. The official websites for all are easy to find and will be noted as the official site.

You have to pay attention when you book things too as the middlemen often have confusing web presence. I accidentally booked Viator for a small local tour in another French city. I was surprised at the price because I had already found the tour earlier --- turns out I somehow booked viator instead of the vendor. WAs able to cancel and re-book. Be sure you are on the official web site and not one that has paid to advance its listing ahead of the official sites. It is not always obvious.

We want to prebook for window 10/1-10/4 but also want to adjust to weather forecast a few days out. Say trade Versailles for Orsay in case of rain. Which option is the most practical, flexible;
book now on venue website
Book viator and cancel if rain predicted
Wait to book till 5 days out.

Waiting for Eiffel might not work at all……….

Also plan on getting to Versailles ourselves via N train from montparnesse…….practical?

Is there any special reason you want to go from Montparnasse? That way is fine, as long as you know that the "N" train goes to Versailles Chantier station, which is further from the Chateau than Versailles Rive Gauche which is served by the "C" train. You can transfer between the lines at Viroflay.

Montparnese since we start out the day at the Pullman Montparnese and have to return thereto collect bags to transfer hotels that evening.
I see what you are taking about.

Can you buy one return ticket on the N/C combination from Montparnese?

Thanks

You actually 2 tickets between Paris zone 1 and Versailles Rive Gauche. Cost is 4.05 EUR each. You are free to choose your route.

you take the metro to the nearest C station and transfer to the C there for Versailles Rive Gauche Chateau -- about 4 Euro each way.

If you are starting at the Montparnasse Pullman, you might look to see if there is a rear exit on Rue Vercingetorix. Then you take a left on that street and walk about 100 feet to Avenue du Maine and look to the left and you will see a Metro entrance sign. That is line 13 and you take that to Invalides to change to the RER "C" to Versailles Rive Gauche. RATP gives that as an alternate route.
https://www.ratp.fr/en/itineraires#no-back

So I’m confused …..2 tickets required?
Buy Metro ticket to RER line C and buy another ticket there. I presume that the Navigo easy pass filled with 10 metro tickets does not work even though the price is the same (see above)

No, you buy one ticket to go to Versailles Rive Gauche for 4.05 EUR and one ticket to go from Versailles Rive Gauche back to anywhere in Paris for the same price. The reason for buying 2 tickets is that you don't have to wait in line at the ticket machine in Versailles Rive Gauche to buy your return ticket.

After you buy the ticket, you scan it to open the turnstile to the Metro, then scan it again to enter the RER station, then scan it again to exit the RER station at Versailles. Repeat the process on your return with the other ticket.