Barbra, Yes you will need to renew your passport, but you certainly can book your tickets now.
But, am I reading correctly from your last post, that you have already booked with Flight Centre? I hope you didn't.
The seat selection problem, is a common occurence when you don't book directly with the airline or airlines in this case. It's one of the reasons everyone suggested you book with Finnair or Air Berlin or whoever, other than Flight Centre. With the Brisbane to Singapore, it is Finnair but on a Qantas codeshare, right? (correction, it is Qantas on a Finnair codeshare, right?) Then Qantas should be able to help if call and request, but I might be wrong?
I think Mattg's advice has been awesome. You should read his Trip reports, almost a european expert. The Germania flight looks good. Also I remember when searching Finnair for you yesterday, that other than Rovaniemi, there was the option to fly to Kemi ? It's about 118kms away? Too far? Just an option to consider.
Hope it works out.