Please guide on priority.....

Hello Gaurav, and welcome to the forum. As for your questions: the timetable helps you pick the right (night train) journeys. Just fill out your departure station (such as Venice), arrival station (Lucerne) and the right date and time to see all possible connections. You can plan your other trips in Switzerland the same way. It also shows traveling times, which is a great aid in planning your day trips. Generally, I'd say that you should take one hour at least up at every mountain. There's quite a lot to see and do at the Jungfraujoch, so that could last about 2 hours.

Costs can be found through the timetable and at the several mountain transportation companies' websites. In your case, a Swiss Card would be worth considering. It provides a free trip from the Italian-Swiss border to Lucerne, back again from Interlaken to the border on your way to Spain, and a 50% discount on your remaining trips. Order options are listed in the upper right corner of the Swiss Card page. Since you're in the Netherlands now, you can select that as your current country of residence.

You don't have to book tickets online, but you can do so if you want along with your Swiss Card. The only trains that require seat reservation are the international ones crossing the border. You can read more about it at the seat reservation page.

I hope this helps you out!