Sorry, I'm a bit late to the party. But you still have 7 months to go, so I thought I'll give a shout.
This is the first I've heard of EuroRoutes. They're definitely not too well known around here (Finland). We do have national cycle routes, here's a reasonably good web page for information on that: http://www.pyoraillensuomessa.fi/ind...sis&pid=2&l=en
Judging by your destinations, it would seem you'll be riding from Helsinki towards Turku / Åland isles, then Stockholm, then south towards Denmark? You will need the Finnish cycling GT map number 1, which, unfortunately, is out of print right now. Been trying to get one myself. They're running a new batch come spring, but they don't have the exact date nailed down yet. Here's the map shop: http://www.karttakauppa.fi/workspace...e=true&theme=5
. You can get by using normal GT maps, but those are less detailed and do not show the designated cycle routes. I would definitely prefer the cycling map instead.
Turku Archipelago trail
and the Åland isles
are not to be missed. IMO they provide some of the best cycle touring scenery in Finland, with nice terrain, good roads and very little traffic. If you want to spend a rest day or two there, I can give you pointers to my favorite tourist islands there.
You won't log in a lot of mileage in the Archipelago/Åland stretch, it's basically island hopping from one small island to another, connected by ferries and terminating at the autonomous Åland main island between Finland and Sweden. From there it's just a short ferry connection to Sweden. I'm not sure if the GT1 cycling map covers Åland, but they have their own cycling map available. I don't have the details of the map with me, but I'll have a look when I get home.
About ferries, some of them run continuously, some of them run a couple of times per day, and some of them run a couple of times per week. Main routes are well served, but you should educate yourself about the connections you will be using, to avoid any surprises. Whichever map you use for the Archipelago/Åland part, make sure it has ferries marked on the map.
 uhh, forgot: "wild camping" is perfectly legal in Finland in rural areas (which we have no shortage of). In Åland isles it is "restricted" meaning they want you to support local economy through the use of commercial camp sites. Nothing wrong with that, but they might as well say it out loud.
From what I've seen, you can get by wild camping there as well, just keep low profile and stay out of sight. [/edit]