Hi Chris..thanks for clarity about the pass coverage.
You've done your homework.
One precaution. Keep an eye on the weather at places with webcams and links noted here.
en.swisswebcams.ch/suc he/ort/m%C3%A4nnlichen -6935774/
Weather in the high Alps is controlled by the mountains....predictability is low. It definitely comes in layers. It can change very quickly. We went up to Wengen in clouds and fog in June in 2015, and at Wengen, the sun suddenly appeared and the LB Valley was clear and beautiful. 3 hours later, it closed down again. The sunny stretch had not been predicted.
The last thing you want to do is count on the Jungfrau and then find it is cloud covered. I have been at Männlichen in the sun, and seen the valley below completely socked in. August provides a dilemma. It is best to get tickets at the last minute to know wbaout the weather...or perhaps the night before, but in August, the trips all the way up are packed, with lines. if you go up to KS and try to get a ticket. Especially on nice weekends, but as you demonstrate, August is holiday time for everyone.
this link and links on that page can help with weather.
The train station and tourist info bureau at Interlaken can give you a last minute report, in detail. Same at Lauterbrunnen or Grindelwald.
If you are not going to do the Jungfrau, which I recall might be the case, the precautions are not so critical. The cog rails from LB or GW up to KS have longer trains- more cars- when the crowds get heavy, and they run more unscheduled trains in between scheduled times to handle the load. KS to Jungfraujoch is the bottleneck.
<<"we could just train it through to Lauterbrunnen and Grindelwald (not sure what the time differences are between Train and Car for that short stretch?) But it will mean less time for me driving if we can hop on the train at Interlaken.">>
The train trip is not terribly long. 20 minutes from IO to LB. Going by car takes a similar time...maybe a cou[ple of minutes less unless you get behind a slow driver. However, the trains only run every 30 minutes. You have to park your car and get to the platform. If the small parking lot is full, you may have to park a couple of hundred meters away.
Worst case just miss a train., and have to walkback totthe larger parking area...or wait 30 minutes.
A twenty minute train ride can become a one hour train ride.
Parking at LB is integrated with the station.
The train station and tourist info bureau at Interlaken can give you a last minute report, in detail.