OK although I agree the GBC guilty theory seems the most obvious outcome, I have some alternate ideas about some of the assumptions people are making:
Firstly as far as I gather the daughters were at home on the night - I think the sleepover at Senior BCs on Thurs night is a furphy stemming from a misunderstanding / misinterpretation of ABCs friends comments which I take to refer to a planned sleepover on the Friday night (if that makes sense!?).
So (for an alternate theory) on the night, daughters at home in bed. Mum and Dad working hard to patch things up, but break into an argument about something (previous affair, finances, maybe even feuding in-laws?). GBC storms off to his (possibly separate) bed leaving ABC fuming on the couch watching the Footy Show.
ABC then contacts a 3rd party and gets in the car late to meet them - possibly to confront a GBC lover, maybe she had a lover herself, maybe there is some other connection eg bad finance dealings.
This 3rd party commits accidental or intentional murder for unkown reason and covers it up, later in the night sneaking ABCs car back into the carport.
This explains Police frantic search initially for a missing person, and their interest in car movements through the wee hours.
GBC wakes up feeling remorseful about the argument, can't find ABC, rings and texts the obvious friends/family and for some reason starts to think the worst, maybe even having good reason to suspect a 3rd party and/or foul play. He's got the girls, so he can't leave the house and so he calls the police
The Police search begins and we know the rest. To possibly explain other events:
Lacking other leads, Police have focused on eliminating GBC by forensic checks on home, car, computer - they need to do this given he is the obvious/easy one for the tabloids and angry mobs to focus on and unless he is completely eliminated he could provide a potentially convenient way for the real murderer to shed reasonable doubt at a future trial.
GBC not joining the search / manning the command post - Police would not allow him to search for above reason, polluting the crime scene. And he wanted to be with his daughters and keep them away from the command post in case of a grisly find.
His unusual behaviour and lawyering up - I read nothing into this, who knows what was going through his mind or what is a normal reaction in this situation (guilty or not)! As a real estate agent though he would have had regular dealings with lawyers and is reasonable he would turn to them for advice early on. But a smart lawyer would soon realise this could blow out of proportion and recommend an expert criminal barrister be brought in when ABCs body turned up, given the husband is generally the first and most obvious suspect in these cases (as evidence by the overwhelming voice of opinion on this site).
The car crash - was he being persued by a tabloid journo or photographer and tried to evade them? Again given all the speculation about guilt it is no surprise the police later impounded the car to test. These actions are as much about methodically ruling out all theories, as proving guilt.
Who is the 3rd party, the suspect in this theory? At this stage I have no real ideas on this but comments by Police may indicate GBC has a good idea but the Police are having trouble putting evidence together? Or maybe GBC is not saying as he's worried about future reprisals eg daughters meeting foul play?
The triad/bikie thing seems far fetched but not impossible!
