He signed in the imperial manor as Michael and not Michael Alexandrovich. No emperor signed with their number such as Nicholas II. Therefore I feel Michael thought of himself as the container of imperial power when he signed the document and hence the style. I would welcome other views mind you
I really don't know what you mean by the term "the container of imperial power". We can really only speculate at this point about how Michael felt about all of this, unless someone finds a document Michael wrote that says "I was only a caretaker at that point", etc. What historians can reasonably do is to point out what characteristics he had as a person and figure out if the description pertaining to him fits or does not fit. I'll give you an example, although I believe there is more detail in my biography of Michael on the APTM.
Kerensky says in his memoirs that Michael decided to turn down the throne because "we could not guarantee his physical safety". This does not fit with what we know about him or the Romanov dynasty:
1. Because of the political assassinations, members of the dynasty were well aware that they could be targets, too. Would any of them even believe assurances of physical safety? Doubtful.
2. Michael was awarded medals for physical bravery, so it does not make sense that he would sacrifice the throne because he was scared.
3. Michael's diary that morning clearly shows his greatest worry at the time was for "justice and order", not that he could get away from Petrograd with his skin intact.
I've already said that Nabakov's presence was significant - in my opinion, bringing in the acknowledged expert on the Fundamental Law to the discussions with Michael clearly shows he was concerned about the legalities of his situation.