Here are partial answers to some of your questions.
<<"Alternatively, we are planning to drop Chur and visit Bern instead and stay at a pretty BnB near Signau station. Please share your thoughts on this plan.">>
I stay in the Emmental regularly. I used to stay at the Gasthof Bären Signau. It is a famous old Swiss country inn. It has changed hands a couple of times, so I can not comment from recent experience. It was a good value. It is directly by the train station.
Current room prices are in line with Emmental norms.
The rooms were clean, neat, and nothing special when I stayed there.
Don't know about B&B's or relative prices. Inns (Gasthofs) usually include a basic breakfast in their room cost.
Signau is a nice town to walk through. The main street has lots of Emmental character. All of the Emmental has good walking, with well marked trails.
If I were to stay in the general area today, I would stay in Affoltern im Emmental, at the Landgasthof Sonne. However, it is awkward to get to by public transportation. It is also further from Bern in time and distance. I have stayed there without a car, but it much better to have a car to access it.
Not so far from Signau is a very high quality in Langnau im Emmental. It is the Gasthof Hirschen.
I have stayed there many times, and recommend the inn and the kitchen. It is a few stops further along the train line from Bern.
It is definitely one of the better Emmental inns.
The prices are a little higher, and slightly above average for the region, because it is a very good inn.
It is a couple of hundred meters easy walk from the train station in Langnau.
<<"We do understand that Swiss weather is a little unpredictable, however, would still like to know the weather during last week of April and first week of May.">>
In the lower altitudes, near Bern for example, Spring will be well along by the first week in May.
I have attached a picture I took near Sumiswald (a few miles from Signau) in mid-April 2014. (On April 17th.) April 2014 had an unusually dry, warm and sunny week or so in the middle of the month. It was sunny and dry with mild temperatures for about 10 days in a row, with only one or two rainy days. More like May than April. I usually expect frequent rain. However, the weather is highly localized, and often the rain will be in only one region.
In the picture you can see that the grass is not yet really green. The trees have flowers but have not put out leaves, a few fields have been plowed and planted, and the Alps have a lot of snow left.
If you are lucky, you'll have some nice days like that.
In the Alps it will still be winter, or between seasons. There will be lots of snow, and only certain trails will be open for walking. It will be cold, unless you have a clear day with intense alpine sunshine. Since skiing will not be so good by May, many facilities will be closed for maintenance or holiday before the summer season which begins at about June 15.
The difference is due to altitude. The weather varies a lot with altitude.
<<"And is food very expensive in Switzerland? We are even ready to prepare quick food of our own eg: sandwiches.">>
Expensive compared to what?
Bakeries all sell sandwiches. The convenience stores (Kiosk, Apero, Volg) at most train stations of any size sell sandwiches.
Most inns and restaurants offer one "low cost" meal each day...the "Tageskarte" or "Menu" that is typically around 25 CHF. Maybe 3 courses - a soup, a main dish, a dessert, for example.
Through out Switzerland COOP stores and Migros stores sell groceries and prepared foods. Normally in larger towns. You can get a lot of foodstuffs at those stores.
Most first time tourists are not lucky enough to visit the Emmental. The heart of Swiss farm country - real Switzerland, not flooded with tourists from other counties. Enjoy it!
Here is a thread with some information about the Emmental, including lots of photographs. Scan down the thread.