I did say “... to some degree” because there is good reason to restrain and accidentally suffocate her in the scenario where another person is in the apartment checking to see if MM and her siblings are okay - immediate danger would have required an immediate and decisive response. IMO, the reason to kill her when no one was in the apartment lessens because the risk of being caught is significantly reduced - this seems to be what you have expanded your theory to include. Plus, getting away from risky situations is consistent with CB’s known previous behaviour.
As I mentioned in my initial response, the part of your theory which troubles me most is that CB had to have made a judgement that taking the corpse and carrying it through PDL in the evening was less risky than leaving it and getting caught forensically. That or he panicked and took the body without thinking which I don’t think fits his career criminal history. Both options seem unlikely but the case has remained unsolved despite 13 years of expensive investigation by two and now three police forces - we probably should expect the illogical.
I simply think that in your theory, it makes the most sense that CB was in the apartment during MO’s check. At this time, he accidentally killed MM while avoiding detection and did so in a manner (suffocation, silently and leaving no evidence, I still think this is problematic) which didn’t arouse any suspicion from MO - as mentioned previously there are supporting factors from MOs statement and subsequent “disturbance” comment. If this occurred then it fits the timeline that CB left on foot carrying MM and was perhaps spotted by the Smiths. Again, exiting with the corpse IMO seems unusual but is still possible given some of the reasons you’ve put forward.
I'm not committing to any particular sequence of events of what took place in 5A at this stage, just giving some alternative options, most of which I've mentioned before.
At the moment, the only points of my theory (if you want to call it that) which I'd commit to are -
1. CB didn't enter the apartment with the intention of kidnap but for some other reason (burglary or abuse in situ).
2. While in the apartment, CB attempted some form of abuse upon MM.
3. MM woke up and CB killed her before taking her body away from the scene.
As for the hows, whys and whens, I am only giving options and am yet to convince myself on any one specific sequence of what played out at present. There are a number of scenarios that I've mentioned which are plausible though as far as I'm concerned. The same goes for the one you are favouring.
Just touching on the point a few people have made about why CB might have done such a thing, or why didn't he just run etc. It must be remembered that we are not talking about a normal mind here. What might seem logical or rational to us, is not necessarily the case in someone like CB whose motives and deviances are difficult to comprehend.
It also can't be ruled out that he was not of sound mind at the time. Under the influence of drink and/or drugs, even people with no prior history can commit the most heinous of acts, things they would never do while sober.
As for being in keeping with his MO of previous crimes, I'm not sure which ones are comparible to this hypothetical scenario. The circumstances of the ones we know about all have different variables. He alledgedly ran from the scene with the schoolgirls, but not did not run from the playground for example.
The crimes most similar to the scenario being proposed here are the sex attacks on young british girls in their beds across the area in the years prior. Admittedly there is no evidence that these were commited by CB, but we can possibly learn something about how such an intruder might react to a victim waking up.
What we know about these attacks is that several of the girls did wake up. They were able to describe the intruder and in some cases the abuse that happened (implying it may have continued after they were awake). We know that in at least one case, the intruder spoke to the girl and tried to placate her. I can't find many details of exactly what happened after the victims woke, how long the intruder stayed, whether they screamed, how the intruder exited etc. But it appears the response wasn't to immediately run away in at least some of the attacks. And this would have been where the parents were in the same apartment, where the logic of immediately fleeing is more probable than if the child was alone. In other words, for him, the urge to continue the abuse outweighed the risk of the now awake child potentially alerting someone.