I think it is important to keep in mind that the Revolution was an extraordinary time, and the rules were tossed up in the air. As has been mentioned, Nicholas II legally couldn't abdicate for his son, but he did, and it was accepted (why wouldn't it be accepted; Nicholas was an autocrat, so could change the laws as he saw fit, provided he had the backbone to do so). Was Michael tsar for an hour? Yes...and no (IMHO). Nicholas DID pass his powers on to his brother and "bless him on his accession to the throne..." and later Nicholas sent a telegram addressed "To His Majesty Emperor Michael." Upon arriving at Petrograd from Pskov, Shulgin proceeded to proclaim the accession of Tsar Michael II. I've also read that elsewhere in Russia Michael's accession was also proclaimed, and Nicholas's portrait was replaced with Michael's. Clearly, many people believed Michael to be tsar immediately upon Nicholas's accession. Also, Nicholas and others referred to Michael's "proclomation" as his abdication. Clearly, Nicholas believed his brother to be the next Tsar of Russia. Furthermore, in his proclamation declaring himself empreor, Kyril stated "The Russian laws of Succession to the throne do not permit the Imperial Throne to remain vacantafter the death of the previous Emperor..." I would think if anyone knew the laws of succession it would be Nicholas II and Kyril.
On the other hand, those were extraordinary times. Clearly, the provisional government did not want to acknowledge him as the new tsar. Yet, they wanted him to renounce it (if he wasn't tsar already, what was he renouncing?). The result was his proclamation, which states "...by the will of my brother, who has transferred to me the Imperial Throne of all the Russias..."(emphasis mine) but then goes onto state that he will not assume supreme power uless it is the will of the people. It seems to me, he is stating "the imperial power has already devolved on me, but I will not excercise it unless the will of the people." This was almost universally hailed as his abdication.
BTW, according to at least two of my books, Nicholas DID sign two abdication manifestos. The first one was prepared before his discussion with Federov concerning Alexis's illness:
Nicholas and Alexandra, Massie, Robert K.: A form of abdication, prepared at Alexeiev's direction and forwarded from Headquarters, was produced. Nicholas signed it, and the document was dated 3 p.m., March 15. The throne had passed from father to son, as prescribed by law. His Imperial Majesty Tsar Aalexis II, aged twelve, was Autocrat of all the Russias.
The Fate of the Romanovs, King and Wilson: The succession laws of 1970 ditated that the throne should go to Alexei. In the early afternoon of March 2, 1917, this is exactly what Nicholas did, abdicating for his twelve-year-old son...Having signed the manifesto, the former emperor could do nothing but wait....changed his mind, abdicating a second time, for himself and for Alexei. (sorry for condensing it, but did not feel like copying the entire paragraph plus, but that is the gist of it).