Hej suomenlainen, good question-you have done homework very well. On the old-style paper passes this was no problem (except that you had 1 day less to use trains overall).

IF you go to Eestti: forget the discount, it is very small. Also think again about even using pass to go from there onward: its hardly worth it and the trip wil be very cumbersome-normal people go by bus there. Best overview on how to is on seat61.com (ith the complete trip to UK).

IF you go to Sverige: its Viking or Silja: I think best to call them-maybe they make no prob about it at all. And I guess you are junior? In most cases the discount is then also by age and the same as for passholder, so that saves it.

This is actually a good question for customer support: with a mobile pass, how can you let the validity period start without actually using a travel day? Since indeed this is possible with a paper pass so in my opinion the mobile pass should also have this possibility.

Hello, I realize that this answers comes likely too late to be useful, but just in case other travelers were wondering the same thing: any benefit, including discounted ferry passage, may only be accessed throughout the pass’ validity. That means that you can only access the ferry benefit with an activated and valid pass. Please note that travel days do not have to be used for discounted tickets, only for ferry passages that are included in your pass. 

This addition is also too late but for any future travellers with a flexipass: you can activate the pass and then cancel the first travel day that is automatically activated with the pass activation. Wen the app then asks whether you want to deactivate the pass, you choose No. And then you have activated the pass without using a travel day.