after all the scenarios hashed through i still believe the simplest....
allison went to hair salon
drove home
was looking forward to night without children
was looking forward to a busy next day (conference)
had argument, dv
maybe police called (by who, allison, neighbours?)
had a bath
was strangled and drowned in bath, scratching the killer on the face arms wrists chest etc
killer may have dressed her in walking clothes to fit in with out walking scenario
killer drove her to kholo bridge, pulled in to the side of the bridge and possibly rolled or dragged her body to up high under the bridge
killer probably worked alone, running on adrenalin
killer called police 7.30, maybe triple 0, maybe hinting wife may be suicidal, depressed etc, stormed off, hasnt returned from walk etc
police alerted by scratches, story of depression, previous nights call out etc so made it an urgency to search and also suspicious so declared the home a crime scene
killer may have confided in father or sister as burden just too hard to bear, or may have hinted she was depressed and left during the night, upset etc and advised to hire lawyer straight away as husband always suspect etc
i still think the killing was a solo act and cant see where he would need others help except for advice or comfort after the fact.
i was also wondering, if he didnt act alone and the two cars were witnessed at the scout camp area, where there were reported drag marks, maybe that was where the body was going to be left but they saw the witness's vehicle and panicked they had been seen so went with plan 2 and then drove her to under the bridge
i think this was domestic violence with a fatal ending and a big cover up, but think there may have been some pre planning, at least in the killers mind, with a little bit of research into suitable dumping grounds etc, and a tidy cleanup afterwards.
all my opinion only