Berner Oberland pass and training to Italy

You will only be able to use the Berner Oberland Pass to Domodossola if you go via Kandersteg. About half of these trains actually terminate in Domodossola, so you will have to get off anyway. if the train terminates in Brig, you could get tickets on the EC train to Milan by showing your pass at the ticket office and get credit for the portion to Domodossola.

From all that, it is not valid on the fast route from Spiez to Milan that uses the Loetschberg Base Tunnel. So you cannot get the EC train to Milan from Spiez. The route via Kandersteg is slower, but much more scenic. For June 7, get the 12:12 departure from Spiez, arriving Domodossola at 1:54 pm. You will have to wait an hour for the next train to Milano Centrale. That is a regional train with no reservations, arriving at Milano Centrale at 4:35 pm, then connecting to the regional train to Varenna at 5:20 pm, getting to Varenna at 6:24 pm. Cost is 15.20 EUR from Domodossola to Varenna. you will have to leave Gimmelwald at 10:30 am for this itinerary.

Are you staying 5 nights in Gimmelwald, or 6? If 6, you can use the 6-day BORP either on your arrival day or departure day. If you use it on your arrival day, then it will be expired on your departure day. Then leave Gimmelwald at 6:30 am and get the EC train direct to Milan from Spiez at 8:05 am. That will get you to Milan by 10:40 am, but you have to wait until 12:20 for the next train to Varenna, time enough for an early lunch at Milano Centrale station. You will get some credit for your Half Fare Card on the Spiez-Milano leg.

To answer the more broad question about the pass in terms of routes that are partially covered by a pass and partially not....

If you're going from Point A to Point C (passing through B), but only Points A & B are covered by a pass, then you purchase a ticket from Point B to C, stay on the train and if someone comes through checking tickets while you're in the Point A to B range, you show your pass and then once you've passed into the range between Point B & C, if someone comes to check tickets again, then you show your paid ticket. No need to exit and reenter the train if your route doesn't require it.

Ticket checking is random. Sometimes you can be on a train for hours without ever getting checked and then sometimes you could get checked twice in a short period of time. There will be times when you purchase a ticket and it never gets checked at all. Then other times you might end up showing it twice.

Also, in broader terms of the BO pass, do you have a list of the activities you're hoping to do? Unfortunately a few things will still be closed during your time there (Allmendhubel, Schynigge Platte, Brienzer Rothorn and possibly the trail from Mannlichen to Kleine Scheidegg). However, as you said, the convenience of having a pass is nice, especially since you're staying in Gimmelwald where you'd be constantly having to buy cable car tickets to go anywhere, including Murren where you have access to the grocery store, restaurants, etc. Gimmelwald has very limited options, so if I were staying there I'd want a pass to not be buying cable car tickets to/from Gimmelwald multiple times a day :)

Thanks so much to the both of your for your very helpful replies. Wanderweg, yes I was hoping to do the higher altitude hikes and stupidly it didn't occur to me that some would still be closed in June due to snow. We will just have to adapt and I'm sure there will still be no shortage of great views. And Sam, thanks for walking me through that train schedule. Our hope is to do the Lake Oeschinensee hike (hope that will be open), thus we will do the train to Kandersteg with good scenery to get there, so might just opt for the faster EC
train to Varenna even though we couldn't use the BO pass. In that instance I might just buy the 4 day BO pass, and just pay the half fare price to get from Gimmelwald to Lauterbrunnen to catch the train and arrival and departure time. Thanks again!