The "Trainline" app uses the DB system to look up prices (and issue tickets) for SBB. So the quotes you get use the SBB international tariff.
For trips within Switzerland it is better to use the SBB app. If you book between 1 hour and 1 month in advance you can even get Supersaver tickets that way. If you plan on travelling a lot in Switzerland get the Half Fare Card.
A two country pass France/Switzerland would not be a good buy. You would still need to get reservations for the Paris - Basel (or wherever you are headed) TGV, and these you can only get at the station in Paris... just book the TGV on line, and buy regional French tickets at the station.

A better option for Switzerland than an eurail pass is the saver day pass. This is a new product, and you can get it here:
https://www.sbb.ch/en/travelcards-an...-day-pass.html