<<"But by train is really not flexible and maybe will miss a few fantasticlandscape in the way...As for the other two lakes, are they possibly reached by car or train? Or must transfer by cable car?">>
Most people find the rains to be better for scenery, even more so in the mountain regions. Their routes are often through places where cars cannot go ...no roads. And, everyone gets to look; the driver does not have to keep an eye on the road. There is no shortage of scenery on the rail roads, Ludftseilbahnen (Gondola car lifts) and other kinds of "ski" lifts available around the Jungfrau region. Of course, if the road permits stopping, you can get out of a car for photographs. There aren't many trains left in Switzerland where you can open the windows....but the ones around the Jungfraujoch do allow that for good photographs.
Swiss trains between major cities are now almost all on twice per hour schedules. The trains in the Jungfrau region are scheduled for every 30 minutes. Maybe every 20 in high season (now.) But, in practice, once the trains that go to and from Kleine Scheidegg and Lauterbrunnen or Grindelwald fill up at, for example, Layerbrunene), another one is usually available within a very few minutes. The company that runs those cograils does not make money if people wait at the station.
I do not have personal experience on the line from Kleine Scheidegg to Jungfraujoch.
I have read that at this time of year there can be waits for a train; the seats are sold quickly.
Arno and Annika have commented on that elsewhere; perhaps they are the best to answer that question. on those details.
The highway driving is usually not a problem even if you only speak English.
However, if you get into a city, where the roads are rarely on a grid, and the intersectios come up quickly, you'll find an ability to read street signs quickly in German to be useful. Even witha GPS, although they minimize the problem. Since you are coming from Heidelberg, perhaps you can reead German?
I cannot tell from the thread what other two lakes you are referring to.
Perhaps this map my help:
map.geo.admin.ch/?topic=geol&X=204500.00 &Y=647982.63&zoom=1&la ng=de&bgLayer=ch.swiss topo.pixelkarte-farbe&layers_opacity=0 &layers=ch.swisstopo.g eologie-geocover
A careful study of the timetable will give you an idea of the flexibility and speed of the public transport system: