Jungfrau train - can hop on and off at any station

Hi,

I would like to enquire on the followings as I couldn't find much info on these for Jungfrau train.

I would be departing from Interlaken to Jungfrau, and understand from the website that I can go up via Lauterbrunnen and come down via Grindelwald, or vice versa.

If I buy return ticket on the same day, am I free to hop on and off at any station along the journey?

If no, how does it work? At every station that it stops by, I couldn't get off? Or I can only get off for few minutes and have to board the same train again?

If yes, I can stay at 1 particular station for as long as I wish? Is there fixed seats allocated? What if one of the train is overloaded?

Also understood from the website that weather is important, however, would the tickets be sold out if I only buy on the day itself? Is the ticket fast selling in June?

Appreciate the advice as I couldn't decide how long would I need and what time should I depart.