Petr., your post reminds me that there's something odd about the Romanovs being held in captivity beyond, say, June, 1917. At first, right after the March revolution ,a revolutionary Government would have a natural interest in holding Nicholas and Alexandra, namely to investigate them and N's regime for whatever they thought may have been the regime's crimes. But that special investigating commission didn't find anything criminal or treasonous for which to try either N or A. And they looked very hard and thoroughly, (as Anna Vyrubova could attest). So, after that point, why keep the Romanovs in custody? The Provisional Gov't, even by its own criteria, could hardly justify keeping the Romanovs captive after its commission essentially exonerated them.
I'm not naive about the political pressure exerted on Kerensky by the SP Soviet but formally there would have been nothing to bar the release of the Romanovs, or indeed, to actively escort them to the Crimea or elsewhere. Like you, I strongly blame Kerensky for not taking the tougher, less popular, but morally right course.