Sorry, the way I read your post there wasn't an actual question. The only thing I felt I could respond to was the vouchers being unusable, which is what I replied to. I did not read your post as you wanting the whole system explained at all. Sorry if my response annoyed you.
You get 5 autographs for the Commander Package, that is correct.
Of these 5 you get 2 assigned autographs. That's one Captain level (or similar) and one bridge crew level (or similar).
Those names are not planned guest announcements, I am just using names for reference. Captain level would mean Kate, Scott Bakula, Patrick Stewart etc. I think Jeri Ryan was also put in that bracket when she appeared last time, and I want to say Nichelle Nichols too. Bridge Crew level would mean Marina Sirtis, Brent Spiner, Michael Dorn etc.
Those 2 are assigned to your ticket. You have those two safe. But: You can't change your assignment. If let's say there'd be Scott and Patrick appearing and you'd be assigned Scott but want Patrick you can't change that.
The other 3 of the 5 autographs are vouchers, that is correct. You can pick any guest charging 20 Euro and use those vouchers on them. I will try to find out if you could use those vouchers on 15 Euro guests, cause from the wording on the website that is not entirely clear. I genuinely have no idea if there even will be 15 Euro guests. It seems 20 Euro is becoming the going norm in Germany.
Destination Star Trek is a bit of a hybrid between the Hub/Chevron style hotel conventions and LFCC. Thus the system looks pretty weird in the first place.
It used to be that way that all autographs were assigned in the Commander Package. That was changed because the attendees wanted a bit more flexibility.
Including all autographs at Commander Level would not work because there's too many guests. Hub and Chevron have 5 to maybe 10 guests. I am aware that The Hub has had pretty large guest lists before, but those guests were UK based and often appearing one day only, thus no significant travel costs etc. that had to be taken into account.
If the package doesn't seem good for you maybe get a Standard Ticket and add on the autographs? That would be the best way to make sure you get all autographs you and your partner really want, unless you want to buy the Admiral package.