If you are travelling with your parents, how far can they walk? I assume you are looking for places to visit with transport, and not long walks in the mountains.
There are two obvious places to use as a base, Luzern and/or the Berner Oberland (Lauterebrunnen, Wengen Mürren, Grindelwald etc.)
Most people would recommend splitting your 5 nights, 2+3 nights. But, for people who can't walk far, I would choose to use Luzern as a base and doing day trips from there, The Berner Oberland needs walking to appreciate.
"1. from paris i should take TGV lyria till zurich/geneva ?" - from Paris, get a train to wherever you want to end up (Luzern?, Berner Oberland?). This will involve changing trains, but just look up times on the SBB website ( https://www.sbb.ch/en/ ), that will find you the quickest route.
"the best mode of transportation" - in Switzerland, always train, except for the few places without trains, and they all have connecting buses from the nearest station. All rail routes in Switzerland have at least one train per hour.
In Luzern you can:
- Tour the town (pretty)
- Boat trip on the lake.
- Trip up Titlis (train + cable car)
- Day trip to Interlaken and back, via the Brünig pass. Scenice train ride.
- Trip up Rigi or Pilatus (two more mountains)
Jungfraujoch is in the Berner Oberland, too far for a day trip from Luzern, but OK if you are staying in the Berner Oberland.
All mountain trips are depend om weather. Be flexible and if it is cloudy, go on a different day.
"return flight to new delhi is from Zurich at 12 noon , should i be in zurich the previous night ?" - no need to waste a night in Zürich. There are trains from Luzern direct to Zürich airport, takes about 70 minutes. If you choose to stay in the Berner Oberland, still possible to get to Zürich in the morning.