Day 1 16th Dec : Land in CDG airport in the evening
Day 2 17th Dec: From Paris Gare De Lyon train station to Zurich
Day 3 18th Dec: Zurich to Lauterbrunnen, explore grindelwald as well
Day 4 19th Dec: Lauterbrunnen to Mt Titlis
Day 5 20th Dec: Lauterbrunnen to Paris
Day6 21st Dec: Paris
Day 7 22nd Dec: Paris
Day 8: 23rd Dec: flying off
You'll have to compare ticket prices vs pass when the former go on sale (quite late due to the yearly timetable change on 15th December).
I'm not even sure Eurail makes sense for your trip. Advance tickets are cheap on TGV Lyria while there's a 29€ passholder reservation.
Then perhaps get the Swiss Half-fare card, a good deal for mountain peaks. But do compare options when tickets go on sale in October (I assume).
If you were to get Eurail it would be a short 4 day pass as the pass is useless in Paris: not valid on buses, métros, RERs.
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