>> 1.Take the boat from Montreux > Vevey Marche > Lausanne From this, we'll explore the city of Lausanne, maybe take a look at Port Ouchy / Olympic Museum / Notre Dame Cathedral / Parc De mon Repos . 2. Take the boat from Montreux > Chillion Castle
Montreux to Lausanne would be my choice for the best scenery, but you will need to check the timetable to see whether there are boat services on your particular date. The winter timetable will still be in force until mid-April, and services will be extremely limited, and not on a daily basis. The spring timetable kicks in around 15 April, but even then there are only two departures from Montreux to Lausanne daily, at 13:45 and 17:09.
(They don't have the 2017-18 timetable up yet).
If your priority is seeing Lausanne, you may need to go by train. If your priority is a lake cruise, I would wait until the new timetable is released, have a look to see what boats are operating on your preferred date, and pick one of them.
>> Take our time to explore Chillion Castle (not sure how long it'll take)
You could spend anything from an hour to half-a-day there.
>> How long is the boat ride along Lake Geneva from Montreux to Lausanne following the route outlined above?
Montreux to Lausanne takes about an hour-and-a-half. You can find out by using the timetable.
>> Q2: However, if I do not take "too long" at Chillion Castle, will it be possible to take the boat from Chillion Castle back to > Montreux > Vevey Marche > Lausanne and explore Lausanne?
Again, you can use the timetable to plan your trip and decide for yourself what's feasible. If you are hoping to do this before 15 April, the boat may not be an option, but you can still get there by train.