susan,
In order to suggest the best train route, it would help to know which of the five Cinque Terre towns you plan to stay in? I've made that trip several times, and will suggest one possibility using Monterosso as the departure point. Also, which town in the Berner Oberland do you plan to stay in?
- The trip from the C.T. to the Berner Oberland will take the better part of day. One of the easiest trips will be a departure at 06:54, arriving Interlaken Ost at 14:28 (time 7H:34M, 2 changes at Milano Centrale and Spiez, reservations compulsory on the first two legs). If you buy tickets while in Italy, they may only be able to provide tickets to the first stop in Switzerland (Spiez in this case), where you'll have to buy tickets at the station for the trip from there to your destination in the B.O. "Switching countries" is of no concern - you'll depart Milan and arrive in Spiez and probably won't even notice (although the buildings in Switzerland look different).
- I wouldn't bother spending time in Milan with such a short time frame. I also don't think you have time for Lake Como. As you'll probably only have time for one other stop in Switzerland, Lucerne would definitely be my suggestion. It's a beautiful city and spending a few days there would be great. There are lots of sightseeing possibilities there.
Getting back to the Berner Oberland, when you arrive at Interlaken Ost, you'll connect with the local train for the short 20 minute trip to Lauterbrunnen. You can either stay there or in one of the other towns on either side of the valley (Mürren, Gimmelwald, Wengen). It will be easier to provide more specific directions once you know where you'll be staying.