Need advice on train tickets in Switzerland

Most of the mountain railways are not included in the Swiss Pass - a discount may apply however.
The Junfraujoch Railway is an expensive private add on.
If you are just doing Paris - Interlaken - Munich I would buy P2P tickets.

David's description is over-simplified, particularly since you're not just planning on Paris-Interlaken-Munich, but have mentioned some side trips during your stay in Switzerland.

I think that either a Swiss Travel Pass or Swiss Half-Fare Card is useful to almost everyone in your situation, and it is a common plan, since ticket prices within Switzerland can add up quickly. Coming from Paris, you can buy a direct train ticket to Basel, which is the most common train connection point as well as the border crossing.

The Swiss pass products all start working at the border and cover you well past Interlaken, to either Muerren on one side of the Lauterbrunnen Valley or to Wengen on the other. The pass also gives you 50% off Muerren-Schilthorn, 25% off Wengen-Jungfraujoch, and either full coverage or 50% discounts on mountain rides in other parts of Switzerland, and covers lake boats, buses, and city trams. Some discount caveats apply, depending on the duration of the pass, but the versions covering 3 or 4 consecutive days are very straight-forward.

See also:
http://www.ricksteves.com/travel-tips/transportation/trains/switzerland-rail-passes

http://www.sbb.ch/en/leisure-holidays/holidays--short-breaks-in-switzerland/swisstravelsystem/swiss-travel-pass.html with link to validity map

and several recent train-related threads in this Switzerland forum.

I think it will be a matter of comparing the options once you have finalized your itinerary with all side excursions.

The Swiss Pass would be more attractive if you were planning to include several other centres - Geneva, Bern, Zurich, Luzern.
An eight day second class pass currently costs CHF363, which is just over CHF45 per day.
As a comparison ...
Single tickets from the border at Basel to Interlaken cost CHF30.
Single tickets from Interlaken to the border at St Margrethen cost CHF47.
And with a pass you would still need separate tickets Paris-Basel and St Margrethen-Munich.

Current prices for Swiss Pass (consecutive days):
http://www.swiss-pass.ch/en/swiss-pass/
Current prices for Swiss Pass Flex:
http://www.swiss-pass.ch/en/swiss-flexi-pass/

This map gives you a clear idea of the discounts that apply to pass holders on mountain railways from Interlaken...
http://shop.jungfrau.ch/skin/frontend/enterprise/jungfrau/images/bergbahn/reductioninfo/swisspass.jpg
Note that Grindelwald, Wengen, Lauterbrunnen and Murren are all included on the Swiss Pass.

This is the current fare pricing for all journeys from Interlaken Ost ...
http://www.jungfrau.ch/en/sommer/tourism/travel-information/tickets-rates/interlaken-ost/fares-for-individual-travellers/
Note the Jungfraujoch offers discount day return (Good Morning Ticket) if you take the 8am or 830am train up, and you descend before 1pm.

Since most of your sightseeing will be in the Berner Oberland region, it might be worth it to buy a Berner Oberland pass (still only get a discount for Jungfraujoch). We bought Supersaver tickets online from the SBB site from Basel to Bern (only a great deal if purchased at least a couple of weeks in advance), then implemented our Berner Oberland passes in Bern. Included free rides for the lake boats on Thunersee and Brienzersee, not sure about Harder Kulm. The "BOB" passes can be used all the way to Luzern, then you could get Supersaver tickets from Luzern to the Swiss border enroute to Munich. Try to price it out if you have the patience - I kept looking and looking at all possibilities and became very tired of trying to get the absolute best price, so we finally went with our plan above.