I agree with the others. The sights that you want to see are in the Lauterbrunnen Valley and staying in Interlaken will add both time and cost to your sightseeing. As mentioned previously, your Eurail Pass will only be of limited use in this case.
Regarding your question on the Jungfraujoch, IMO it's easier to access via Lauterbrunnen. You'll first take the cog wheel train up the hill to Wengen and then Kleine Scheidegg, and then transfer to the Jungfraubahn (bright red, so easy to spot). One point to mention about visiting the Jungfraujoch is that it's a good idea to check the weather as it could be fogged over at the top so you won't see much (although you can still tour the Ice Palace and the indoor parts of the facility).
If you love walking, you'll have lots of choices. There are good hiking opportunities on both sides of the valley, and the area around Mürren has lots of trails. Is there any possibility you could add a day, as you'll really be pressed for time to fit everything in that you want to do.