Lost in Swiss....

Thanks for the advice Rahman!

Renren, all details on the Swiss Pass can be found at the Swiss Pass page. You'll find a link to a validity map there too, just as an up to date overview of purchase options on the right side of the page. Just fill out your country, number of travelers and pass variant to see which option is cheapest. The advantage over the Eurail France-Switzerland Pass is that it covers a trips in Switzerland. If all you need in France is the French leg of your inbound trip to Lucerne, it wouldn't be worth buying a France-Switzerland Pass. You'd be better off with a Swiss rail pass, and a separate ticket Colmar-Swiss border. I'd say you should consider a 3 day Swiss Flexi Pass or a Swiss Card for your stay. Details and purchase options can be found at the pages I linked to, just as with the Swiss Pass.

All trips can be planned using the Swiss timetable.

When buying your (discounted) ticket for the Jungfraujoch, you can best do so at a manned railway desk in Interlaken and specify your route, as this will be printed on the ticket. You could choose to go up one way (via Lauterbrunnen and Wengen) and down the other way (via Grindelwald), thus making a round trip and enjoying both stunning routes. Just ask the personnel and they'll get you the right ticket with the right amount of discount if you show your rail pass. As for temperatures up there: it'll be freezing in March.

Most trains don't require seat reservation. International trains and special panoramic trains such as the Golden Pass do need reservations. You can read about this, and arrange for your reservations, at the seat reservation page.