Teej, I don't think Freeman is jetting back and forth. I think the late filming schedule was a combination of him having to wait until they could release him from The Hobbit, and Benedict finishing his run in Frankenstein (which was extended beyond the initial intended run) So I'd guess they've been doing read-throughs and filming continuously since they both became free in May or whenever.
I THINK it'll be fairly early in 2012; there's a picture, taken on set by Mark Gatiss, of the skull on the mantelpiece in a Christmas hat. So if there's a Christmas episode surely they'll put it on as soon after Christmas as possible? Or perhaps they will even manage to get it on before Christmas? ...
Puppy, I know, the three 90-minute episodes format is a bit odd isn't it? I think they initially thought about doing six or eight 60-minute episodes (the pilot was 60 minutes), but the Beeb suggested they make them longer. I don't see why they couldn't make six 90-minute episodes though. 