Plane tickets booked finally

The Coop is not a department store.

It is a supermarket.

The largest one in the area is at Interlaken Ost train station, just across the road from the station (watch out for buses and taxis as it is the bus hub area). It has a small restaurant, too.

The smallest in the area is in Mürren near the post office and not far from Denise's place. It has all the usual things to feed hiking tourists but the aisles are narrow and the selection is limited.

A bit bigger is the Coop near the station on the same side of the road as the cable car up to Grutschalp and Mürren. It has a considerably larger selection but much less than the full sized store in Interlaken.

For basic salads, salad dressing, bread, trail sausages, trail snacks, milk, bread, breakfast cereal, etc. any of the branches will be fine.

If you don't find baked goods to your liking in the Coop all the towns and villages have a bakery which provides even more choice.

In Switzerland, Lauterbrunnen is most central for transportation to Jungfrau and Schilthorn, right in the middle. It will also be the first place you come to from Interlaken. I think Grindelwald would be my least favorite of the four, not that there is anything wrong with it. Just a little bigger town that is more obviously commercial. COOP grocery stores are located in all 4 towns, no worries there.

But I have to say that there is something special about the mountain villages of Wengen and Muerren. Lauterbrunnen is in a deep valley, so the sunlight hits less than on the slopes of the mountains. Both villages are car free, so no automobile traffic. I've staid in Wengen and it has quite few larger hotels. But when I visited Muerren, I was struck by its quintessential Swissness, what ever that means. As soon as you leave Lauterbrunnen to take the cogwheel train up the mountain to Wengen, or the little cable lift to get the other little mountain train to Muerren, you feel like you are leaving the busy world behind and entering a new magical realm. These towns are more expensive, but that is the way it is. Location, location, location as the estate agents say.

The whole Berner Oberland area is hard to understand until you go there. Lauterbrunnen is in the valley at the base of two mountain ranges one on each side of the valley. High up on one side is Wengen and Grindelwald ( the more touristy option) the other side is Murren and Gimmelwald (the quiet small side with no cars) So, really, any of the places you mentioned you can practically "see" the others. We prefer the quiet side, but the other areas are just a gondola or train ride away. From Murren or Gimmelwald you can almost see all the way down to the valley and back up to the other side where Wengen sits. The way the mountains sit so close together is what makes the area so amazingly beautiful. We have stayed in Gimmelwald twice, but there isn't much there. If you want to go to the grocery store there is a COOP in Murren, and it's just a small grocery store. Or in Gimmelwald you can buy food from the neighbors (milk, eggs, sausage, cheese, etc) but still will need a grocery store for extra's, and there isn't one in Gimmelwald. I vote you stay in Murren. Not a lot to really do there but enjoy the beauty, relax and hike, take transport around. It is my favorite spot on the planet.

Your plan sounds great! On your 'rest' day in Paris, I vote for a bottle of wine, cheese, bread and sit along the seine.