Hey sparksy,
As JC has said, it'll potentially be quite wet in Jan. last year we had a late wet season, so you might be right, but I'd count on there being a bit of water around. As JC has said, just be flexible in your travelling.
7 days for the trip up is plenty, you'll be able to get a good feel for the costal towns by doing that. Bris to Mackey is about 12-14 hrs, and as McGirr has said, another 8 to Cairns, so you should be quite relaxed. Maybe stay for 2 nights at somewhere like hervey bay/Agnes waters/etc, and have an explore. Or a night on Fraser would be nice as well.
As for accommodation in Gladstone/Rockhampton/Mackay , we've never actually had a problem rolling into town late and getting a hotel room, camp site or cabin. I'm not sure if it's as bad as everyone makes out? Maybe we've just been lucky, but we've always had our pick of spots.
As for cairns, there's heaps of stuff to do here. What sort of things are you hoping to see? Ie, reef trips, rainforest, self-drive or tours, etc? Skyrail is a definite must, but even things like the rainforest dome at the casino are great.
Good luck - feel free to msg any of us cairns locals, and we'll help you out - we don't bite!
Matto