Definitely murder. Because there's no other way to explain strangling someone until they stop breathing. That is a long time, despite not sounding it, to keep that hold going and break those bones in her neck. In fact I've heard other pathologists testify that it takes about 3 minutes but even a minute is long if you set a timer and wait.
I agree.
I know it is morbid but I've been reading up on strangulation on wiki and some other sites, there are two different mechanisms that can occur, restricting blood supply or restricting airflow. In this case it appears to be the latter.
If somebody knows exactly how to strangle somebody,
mild pressure to the correct part of the neck stops blood flow and can take as little as 7-14 seconds for unconsciousness, although the victim may regain consciousness shortly afterwards if pressure is immediately released (eg fainting game).
In this case the pathologist stated that the victim showed signs of asphyxia (burst blood vessels in eyes). It requires 6 times more pressure to stop breathing than it does to stop blood flow and takes at least a minute for initial unconsciousness (imagine holding breath) but again the victim can regain consciousness if pressure is immediately released.
Given that there were signs of a hand being held forcibly over LM's mouth as well as 2 broken neck bones, it appears this wasn't a quick and clean strangulation and I agree, it probably lasted much longer than a minute from the start to end. Just one partial breath or gasp taken by the victim will prolong the time to unconsciousness. It may even take several attempts if the victim starts autonomous breathing after unconsciousness.
In both cases this is the shortest time until unconsciousness - not death - and assumes it is a well executed strangulation. Brain cells start to die around 4-6 minutes after loss of blood flow or breathing and about 10 minutes until totally brain dead. Effective CPR has been known to prolong this time to 20-40 minutes.
And for that reason I believe this is murder, there was plenty of time to stop and think. There would even be the opportunity to give CPR after breathing stopped.
I believe at some point DM must have realised he needed to finish the job otherwise the consequence of LM reporting the attempted strangulation would have been disastrous for him and from that point the cover up was in progress even before death occurred.