A few more thoughts/questions if someone has time...
In the manual it says attach to a 'designated anchor spot' so if no 'official' tether location in a vehicle (do these exist?) for swedish-style tethering, then must do aussie style? (Aussie style will work awesome in my parents sentra... arg!... actually, now that I think about it, the sentra would likely be a breeze compared to the 2-door accent we occasionally ride in.).
I haven't spent too much time thinking about rearfacing tethering, but I thought that although there was some overlap (?) Swedish and Aussie protected different things (rebound, over rotation, etc)? So what is Britax trying to achieve? (this is subjective, I would imagine, but I am still interested in hearing thoughts if anyone has them)
(totally my ramblings below, correct me if I am wrong anywhere, friends!)
Aussie style reduces downward rotation, which in turn reduces the amount of rebound. If you don't go as a far down, you will bounce back less violently.
Swedish style reduces the rebound event more directly, by limiting movement towards the rear of the vehicle. Downward rotation is unaffected.
They both accomplish the same thing, but by different mechanisms, so it seems to me that the desired effect must have something to do with reducing rebound.
The risk profile is different, though. Aussie style doesn't really present any additional risk, from what I have read/discussed with people in the know. Swedish style may increase chest and neck loads to the child occupant. For older kids this is unlikely to be an issue. but for a newborn or young infant with their big head/weak neck physiology, I have read concern about the potential for injury.
Both increase the stability of the restraint, couple the restraint more closely to the vehicle, reduce lateral movement in a SIP crash. I can't see that being the desired effect, though, as those are such nebulous and unmeasurable things.