I am back. I want to confirm/answer some of the questions I asked, and then ask another one...

Yes, the Mobilis and the Visite Paris pass both worked well on the Transilien/SNCF train. It was explained clearly in the literatures and ticket sale machines in Paris.

The Mobilis 2-zone pass was €6.6 per day and 3-zone was €8.8.
The Visite Paris pass for 3-zone for 3 days was €23.4

I ended up buying the 3-zone Visite Paris pass for €23.4 cuz it was cheaper than the 3 days of Mobilis pass and I wanted to visit the Montparnasse tower (30% discount = €3.5 off) and Arche de triomphe (20% off = ~€2), so it was by far the better deal.
Also, as expected, I was dead tired by the time I got to Paris, and the weather wasn't perfect, so I rode on various buses almost all 3 days I was there. That was pretty fun too.

(I may be off by a few tenths of Euro in some of the prices above).

Question: I learned (the hard way!) that the visite Paris pass expires at/near midnight. On my last night, I stayed at the Grand Arche (La Defence) too late, taking photos, and was denied getting on the subway. I ended up having to buy tickets. The price for individual tickets were €2.7, while 10 was €20.xx. The next day, I asked the lady at the subway, but she was not able to explain to me the significance or difference of these tickets from La Defence. Why are they so much more expensive compared to individual/carnets at regular Paris subway ticket machines? Do they have special usage I can take advantage of? (in hindsight, I should of brought only 1 instead of 10....)

One interesting tangental story: I not only lost usage of my Visite Paris pass to get back to my hotel beside Asnieres-sur-Seine train station, I missed the last train from St. Lazare. I know there is an ~hourly night bus, but I literally missed it by 15 seconds at St. Lazare station. Not wanting to wait another hour or so (it was 1:20am by then), I decided to take the subway to Gabriel Peri and jog the 1.2km to my hotel. Of course, with my luck, when I got to the station exit, google map on my phone didn't work for some reason, and I have no idea how to get to my hotel. But luckily, after trying to explain my situation to a bunch of RATP employees, and trying desperately to get them to at least point me in the direction of the train station, one of them stylishly tossed his cigarette, pointed at his company car and then drove me to my hotel!

Another unrelated note: MacDonald Cafe Macaroons in Paris are quite good, especially considering their price! (I brought a dozen for €10.) Much much better than the free ones at the Air France Lounge at Terminal 2e. :-)