I read the whole thread, and have a couple of comments-
1.- I think a map would help you a lot with some of your questions. In particular, you can seet hat Grindelwald Grund Station is very close to Grindelwald Station. It is a short downhill walk, of about 1/2 a kilometer, but if you do it with luggage, you'll want to have it on wheels.
The main tourist center is Grindelwald itself.
This very convenient map (map.search.ch) will help you see that, even though it does not list actual altitudes:
If you use the menus on this map under "Points of interest/ traffic", you can turn on a layer with the public transport stations.
You can use this map to easily follow the train route from Luzern to Interlaken and on to Montreux. The train line only appears when you zoom in.
Another map gives more details, including altitudes and hiking trails. It also shows train lines, and you do not have to zoom in to see them (black lines) but the detail may be confusing.:
map.wanderland.ch/?lang=de&route=all&bgLa yer=pk&resolution=5&X= 644530&Y=163840&layers =Wanderland
2.- Either map, combined with the timetable:
will show you that the trains that go to interlaken let you off there, and (if you wish to go onward into the valley and then the high alps) you must change to a different rail line to go to Grindelwald or Lauterbrunnen. If you wish to continue further to Kleine Scheidegg or the Jungfrau, you will make another change at either Lauterbrunnen or Grindelwald to the special cograil train that goes up the mountain.
Hope this helps. It gives just a bit more detail than the other posts. Perhaps you have al;ready figured it out, but I thought it might help.