happy,
Just to clarify, there is no town of "Grimmelwald". You appear to be combining "Gimmelwald" and "Grindelwald".
I agree with the others. IMHO, your best bet is to stay in Lauterbrunnen as it's the rail hub for that area and provides good access to both sides of the valley. It will also make your ongoing journey to Paris a bit easier. Which station does your 07:00 departure to Paris leave from?
To reach the Jungfraujoch, you'll first take the Wengernalpbahn cog-wheel railway from Lauterbrunnen to Kleine Scheidegg, via Wengen. When you arrive at Kleine Scheidegg, you'll transfer to the Jungfraubahn (red cars, easy to spot) for the final trip to the Jungfraujoch.
One other point to mention is that some people may have issues at the Jungfrau, given the elevation of 12,000 feet+. You'll probably find that you're moving much slower than normal, but short visits are tolerable for most people.