Hi Susan, and welcome to MySwissAlps!
I will have to leave some of your questions to others, but will address those that I can.
>> things Swiss, e.g. rescue dogs (my life is dogs!)
This is nowhere near where you will be travelling, but I will mention it - perhaps for a future trip!
St-Bernard museum and dogs
>> alphorn playing
This is common at events in summer, but might be harder to come across in May.
You might turn something up through searching here:
>> cattle and livestock
In May, most cattle and livestock will most likely be in their winter quarters at lower altitudes.
Again, not suitable for you timewise, but if you have the opportunity to visit Switzerland sometime in September-October, an Alpabzug/désalpe (German/French terms) might interest you. This is when the cows (and goats etc), decorated with flowers, descend in a procession from their summer pastures to their winter quarters before the snow sets in. The event is often accompanied by a festival including a market selling local farm produce and traditional entertainment by local people. I attended one at Seewis im Prättigau in early October 2016. You can read about my experience and see some photos if you are interested:
>> alpine flowers
You might be a bit early for wildflowers in the Jungfrau Region, but will possibly have more luck somewhere like the Rigi. You might find some useful information here:
>> cheeses, chocolates
Again, not really convenient based on where you are staying, but the most famous cheese and chocolate factories are in Gruyères and Broc respectively.
You will, however, find shops selling Swiss chocolate and local cheeses everywhere.
>> cuckoo? clocks ...
You might find some useful information in my post of Aug 19, 2017 - 2:22 AM on the following page:
>> Wengen 3 full days starting 14 May (arriving evening of 13th) Interlaken 3 1/2 days from May 17
Your travel dates correspond with the tail end of the low season, so some mountain transport won't be operating, but you still have plenty of options for mountain-top visits.
- Gondola Grindelwald Grund to First
- Bus Grindelwald to Meiringen via the Grosse Scheidegg (from 19 May only)
- Trümmelbach Falls will also be open
- Gondola Grindelwald Grund to Männlichen (reopens 26 May, but if the weather is nice enough, they will probably operate it on Sat/Sun/Mon 19-21 May)
- Cable car Wengen to Männlichen (reopens 1 Jun)
- Funicular Mürren to Allmendhubel (closed 9 Apr to 22 Jun inclusive)
>> Lucerne 3 days from May 20
All the key mountain transport and lake boats will be operating.
>> Being from Australia I cannot visualize the closeness of Swiss locations one to the other and would appreciate your comments on how much we could fit in into a day, not rushed.
Don't be deceived by how tiny Switzerland is compared with Australia! The key thing is not distance (especially not as the crow flies!), but travel time. The mountains slow things down somewhat!
The key to finding out how much you can fit into one day, is to use the Swiss Timetable to plan your trips:
>> Given weather considerations we haven't booked anything, and also feel the Jungfraujosh at this time of the year will be too risky. Even the Schilthorn?
You can get beautiful clear blue sunny skies in the middle of winter, and rain and fog in the middle of summer and vice versa. Although at higher altitudes, the precipitation in winter is more likely to be snow than rain. My profile photo with sunny blue skies was taken in Wengen in December one year.
The key is to stay in the area long enough to increase your chances of getting a clear day for mountain-top visits.
>> WENGEN area: Local walks; walking/picnic Lauterbrunnen Valley and waterfalls.
Walks in the Lauterbrunnen Valley will be fine. Walks at higher altitudes, e.g. Männlichen to Kleine Scheidegg are not likely to be open before mid-June. You can check with the local Tourist Offices for guidance on suitable walks.
>> What are the best subjects and best locations in Wengen for photography?
There is a lookout in Wengen that you should not miss. For directions and a photo of the view, please see my post of Feb 20, 2018 - 12:28 PM on the following page:
However, in May, the green might be more of a brownish-green, as the grass dies off over winter and takes a while to green up again.
I seem to recall that there is also a nice view back over Wengen from the Allmend station - a tiny, unstaffed station above Wengen on the way to Kleine Scheidegg.
>> Ditto for alpine glow mornings and nights would be really appreciated. Someone mentioned near a church?
The Alpenglow is unpredictable, but the lookout I mentioned above is a good place to view it if it is happening.
>> Brienzer Rothorn (but I think the cogwheel will be not operating?).
That's right, the train from Brienz to the Brienzer Rothorn closes for the winter and recommences on 2 June 2018. The cable car on the other side won't be operating either.
>> walks/search for early alpine flowers.
As mentioned above, the Rigi might be your best chance.
>> How can we take advantage of our BO Pass around Lucerne given so much of the town and to the east is not in the BO?
- Train Bern-Luzern - through the Emmental (the Emme Valley, from where Emmental cheese originates). To get this route in the timetable, put in "Via Trubschachen". This is an attractive area of rolling hills with plenty of traditional buildings to be seen (free)
- The pass also covers local transport within Luzern (e.g. buses) but practically everything noteworthy in Luzern itself is easily reached on foot