There are actually three processions on that clip:
1st, at the coronation, where faces are indistinguishable - to me at least.
2nd and 3rd, at the Romanovs tercentenary celebration in 1913. They're not the same procession because military uniforms are of two different types. I guess you mean the second procession, where Alexandra holds a little princess' hand?
Obviously a man directly behind the imperial couple cannot be GD Sergei - he was murdered long before 1913. It seems like it's one of the younger (beardless) GDs, probably George - but I'm not sure.