One potentially important point relates to the whereabouts of Nicholas II.
I've had a quick look but there isn't much about. As far as I can see, from some website (that reproduces some British guy's War diary), the Tsar was probably at Tsarskoye Selo as he was dining with the Tsar and the Tsarina.
OTL, in March 1917 the Tsar was well away from Petrograd, on a train. He knew that he and Russia were in a dangerous situation, but he wasn't in immediate physical danger as he made his decisions.
This is a good point. Does the physical danger scare him into ordering the Guards out to crush it asap, or does it paralyse him with fear?
Of course Nicky might have done a hasty runner to Stavka or somewhere else, but he wasn't exactly famous for quick decisions, and anyway there'd be no guarantee that things would be better elsewhere, once the news of the revolt began to spread. He could well find himself and his entire family in the hands of mutineers.
He might have tried for a hasty runner, but how easy would it be to get to Stavka? Tsarskoye Selo is South of Saint Petersburg so that means he doesn't have to get past the mutineers as it were, but wouldn't the trains be in paralysis with the strikes etc?
What happens then? Do the mutineers just lynch them all out of hand? Do they demand Nicholas' abdication (in favour of whom?); Do they try to issue orders in his name? Or do they take him into the Capital and turn him over to the Petrograd Soviet?
I think, as it's early days of the revolution, lynching is probably unlikely (of course, the blood is up in the early days and rash decisions can always be made) but a siege of Tsarskoye Selo is a possibility. If orders are issued in the Tsar's name, it would imply he's captured and 'agrees' to their demands (with a strong possibility that they're rescinded if he's ever rescued). Was Grand Duke Michael popular still at this time? A Regency to replace the Tsar whilst the country is calmed maybe?
Can the Provisional Government still be formed in the same way as OTL? In this situation, unless Nicholas and Alexei are both known to be dead, Grand Duke Michael (even if he's at liberty himself) has no authority to hand over to it. And even if the same politicos try to appoint themselves as the new government, will a Soviet in possession of the Tsar and his family be inclined to recognise them?
If the Soviet is in possession of the Tsar, all bets are off. If it's the "mob", then politicals have a chance of taking control of the movement, especially if the Tsar appears willing to listen to demands. If the Soviet has him, they've got the power and won't let it go.
When did Lenin arrive at the Finland Station? It was after the February Revolution OTL, wasn't it? Who's running the soviets at this time? Hardliners or a committee that might see killing the Tsar inflames world opinion against them?
And how do the generals react? Do they try to rescue the Family? And will their troops obey (or go on obeying for any length of time) if they do? Could we be heading for a full-blown civil war? This could be a right unholy mess. Thoughts?
Some troops will definitely remain loyal. If the revolution presents a
fait accompli with a captured and 'compliant' Tsar there's a good chance many will go over the the revolution. Otherwise, does Kornilov launch a mission to rescue the Tsar or lift the siege of Tsarskoye Selo? Who could resist this, what revolutionary forces are available?
When was the meeting that the Stavka informed the Tsar that the military wouldn't support him in case of revolution? December '16 IIRC? Was this revealing what had been the case for a long time, or a sudden change of mood?