You have an 8 day pass and and list 8 trips. Why wouldn’t you use the pass for all of them — you’ve already paid for it? You just need to buy tickets for the portions of the trip in countries your pass doesn’t cover. Just the portions between the Czech border and Prague, right? I would buy both these tickets at the train station in Berlin. It will probably be easier there as all the German ticket agents are required to speak English. You might save a couple of dollars by buying the Prague to border ticket for the trip back to Munich at the station in Prague.

Take the overnight train from Munich to Venice or look for a cheap flight for this leg. then cut a day from Venice. You can also take an overnight back to Switzerland, but it won’t get you specifically to Interlaken — you have to make some connections in Switzerland. I would add your extra day to any of your stops — probably Paris, for a day trip to Versailles or spend an extra day for a scenic day trip in Switzerland. I wouldn’t add an extra stop, as I’d rather stay in one place and do more day trips.