I went to Finland this year, but after spending a month trying to sort out the best way to get there with the car, I ended up flying because it was sooooo much cheaper.
From the details above, you've pretty much got the cheapest route (as far as ferries are concerned anyway) sorted out.
But I worked out that it would take 4 days to get there and 4 days to get back.
From talking to some of the M-Sport lads on the Sunday after the event, they all go straight to Germany, so the combined trip that was previously mentioned may be the best way to go.
The hotels in Jyvaskyla are normally fully booked by the teams - and many of them don't look that pleasant anyway, so I'd recommend getting a lodge. We hired a ski lodge halfway up a mountain this year, and it cost under 100 Euro each for the week for 5 people.
I can give you contact details etc if you wish.