Hi Wanderdiva -
Here's some general information.
The weather changes quickly and unpredictably in the Alps, so I always carry a light rain jacket and rain hat with a brim or an umbrella.
Around the villages such as Wilderswil, you may find a place to wait out the rain and have a cup of coffee or tea or a beer.. Your chice. You will find inns and restaurants at the stations or nearby. On the trails that are not in villages, such as Männlichen to Kleine Scheidegg, you're in the open. The shelter is at the ends.
See attached images. for Männlichen to Kleine Scheidegg ( nicknamed the Grannies' Walk)
There are commonly places to grab a bite at the ends of well known trails, especially, the scenic ones where they'll have enough customers.
2.- The mobile phone networks in Switzerland have excellent coverage....in the range of 99 % of the country. They may not be available in the most remote mountain areas or valleys, but you'll have excellent service and choices of networks where you will be. Also the trails have excellent signage:
One caution- the trail signs give time to the destination, not distance. On level trails with no serious photography and no long pauses, I meet the times. However, the Swiss go just as fast up and downhills as on the level, it seems. So, if the terrain is hilly ( none that you mention are hilly) add some time.
Grütschalp to Mürren is modestly uphill, but that's the way to walk it, because you can keep looking at the Jungfrau. Partway along that trail, at Winteregg, there is restaurant, with nice views from the terrace.
In Europe, in general, you look for "toilets." A washroom, unless they see a lot of Americans, is a place where you wash things. You rest in a Restroom, also unless you are used to Americans.
In German, singular amd plural - die Toilette, die Toiletten.
There is usually good signage.
The train stations and lift stations almost always have public toilets. They may have a modest fee to enter- carry a few coins to drop in the slots. 10, 20 or 50 Rappen (cents) rarely 1 CHF. They are common in areas with high tourist traffic.
In the less densely traveled parts of the country, universal free public toilets at train stations have almost disappeared. They are now often locked a good part of the evening and night, or even all day. More of them require a coin in a slot on the door. Vandalism has caused many of them to be closed.
At Burgdorf Station, which is substantial station but not in a tourist region, they closed the public toilets, and signs point you to the ones in the convenience store...an "Aperto" brand store as I recall.
However, the mainline trains have toilets, which are almost (but not quite) always well-maintained. They make an effort to keep them in god condition. Mountain trains don't.
You will find public toilets, most of them free, at Lauterbrunnen station, Wengen station, Wengen lift station , Männlichen lift stations and restaurant, etc.
Restaurants may accommodate you, but the situation does not compare to the USA, where the McDonalds all expect traffic at their toilets.
It is my habit if I need to uses a toilet at a restaurant , to order a coffee or such as a courtesy, often before I use the facilities.In some circumstances, it wlil be obvious that a restaurant would expect non-customers to use the toilets.. For example, the Hotel Waldstätterhof in Brunnen has a very large terrace restaurant overlooking the lake. People eating or drinking on the terrace, have to walk into the hotel to find a toilet. So could you.
Many towns have public toilets. Not universal.