You can reach jungfrau by taking a train from interlaken. I'd recommend getting a Swiss travel pass since you can use it against all the modes of public transport in Switzerland (ferry, trams, buses and trains). And please visit jungfrau, it's an amazing destination, much better than the more popular tetlis.
Yes you can travel from Wengen.See what all is covered here under Swiss pass:-
https://www.myswissalps.com/swisstravelpass/validity
You can travel from Wenger to Interlaken or Wengen to Grindelwald. Etc.
Also For the Jungfraujoch query you have check the below link:-
https://www.swiss-pass.ch/swiss-pass/
Jungfraujoch is not allowed by default.You have to purchase additional excursion tickets.
The cost of the pass is 211 Euro for 3 days.I wouldnt really recommend to buy it unless you are making 10 to and fro trips in a day.If you are making just 2 or 3 onward/return journey,the individual tickets should work out cheaper.