johny,
"but eurailways website doesn't show me anything"
First of all, DON'T use "eurailways" to search or book anything. You can check routes and timing most places in Europe by using the BAHN.DE website (German railway).
I wasn't able to find information on "Interlakenhoheweg". I assume you're referring to Interlaken West? The shortest trips on that route from Interlaken West are departures at 09:05, 10:05, 11:05, among others (time 4H:43M, 2 changes in Bern and Lucerne/Zürich). I'd suggest buying your tickets for your outgoing journey when you arrive in Interlaken.
The trip from Lugano to Munich is going to be a full travel day. One of the easiest trips will be a departure at 07:43, arriving at 16:26 (time 8H:43M, 2 changes at Milano Centrale and Verona Porta Nuova). On that route, I'd suggest buying your ticket from Lugano to Milano Centrale at the station in Lugano. For the next two segments, you should be able to buy both tickets from www.capitainetrain.com (their website requires registration, but is very easy to do and the prices are the same as if buying from the rail network). Note that on the Milan > Verona > Munich segments, reservations are compulsory and these are specific to train, date and departure time so can only be used on the ONE train specified on the ticket. Reservations will be provided with the tickets. The segment from Lugano to Milan is a Regionale so don't forget to validate your ticket prior to boarding the train on the day of travel, or you'll risk hefty fines!
All of the above information assumes you decide to use the route that I described.