Yes - the lack of a clear cause of death at autopsy, due to the extensive decomposition, is a problem in itself. The most likely, and consistent with the findings, would be suffocation/smothering (as opposed to strangulation). The hyoid bone was intact, which is often (but not always) fractured in manual strangulation, either from in front or from behind. But suffocation wouldn't leave any telltale marks after that period exposed to the elements.
A hand over the mouth, while pinching the nose closed - easy peasy for somebody GBC's size. And who's to know that the "lightbulb" injury on his right hand isn't the marks produced by a row of teeth? We haven't seen any photos of that injury.... Maybe even chipped a tooth with a finger ring while he was at it - but I don't know if he wears a ring on his right hand...
And that is my conjecture only....

[/QUI
I am sure this is where the defence will hone in on--in order for the prosecution to prove murder they will need a reasonable hypothesis of the cause of death. I was in court when the Dr Milne discussed his autopsy report. He was thorough and methodical in his answers and certainly would not be drawn into a cause of death. And IMO there will be a problem with suffocation because of the degree of decomposition makes it impossible to prove.
A lot of the questioning by Davis centred around Serataline (Zoloft) levels and its metabolite desmethyl sertraline. From my reading the level in the liver of this metabolite 31mg/kg is quite high (De Vane et al "Clinical Pharmocokinetics of Sertraline" Clinical Pharmocinetics, 2002 41(15) 1247-1266 which lists normal liver concentrations as 1.4-11mg/kg. I acknowledge an overdose by Seratraline not mixed with any other drug is almost unheard of but Davis was very keen to pursue this line of questioning. Davis was very keen to get Milne to name his references. I don't believe this is the last we hear about the levels of Sertraline. I have actually downloaded from a site I have access to a few of Milne's references on Sertraline (a couple are only abstracts unfortunately ) and will try to source the reference you gave a couple of weeks ago, mainly for my own interest as Davis was keen on it at the bail application hearing.
I actually re-read the autopsy report today. It makes very grim reading. Davis certainly highlights Allison's long term depression in her medical history and long term use of Sertraline. Despite giving a number of references and ranges he states there is insufficeint evidence to attirbute the cause of death to alcohol and/or sertaline. However he also concludes that it can't be excluded. And there is Davis's out. However remote, he may continue this line of questioning particularly if the metabolites levels are quite high according to some references. He did state at the committal that the alcholol levels often rise as a result of the decomposition and not much weight could be placed on those levels.
I do however, agree with you that suffocation is the most likely cause given all the circumstances that have come out since Gerards arrest.
However, it is interesting the other possibilites he does not exclude. Strangulation is a possibility if "a soft broad ligature was used. It seems like the Bonds pullover is a possibility for this. Drowning was difficult to find signs of because of the degree of decomposition but it is not excluded. He also gives injuries as a result of a fall from a height as a possibility, It seems it is more of a possibility if Allison hit the water. There could have been some movement of her body by the water and a fall could have led to her drowning.