What happens if in 1827 the Decembrists in Sankt-Peterbourg manage to force Nikolai to abdicate in favor of Konstantin, likely by a greater degree of support from the mob and various other army regiments? Will Nikolai attempt to flee the city and govern from Moskva? Does Konstantin agree to take the throne, and if so, does he institute the liberal reforms which the Decembristi called for? What is the reaction of the much more radical groups of Decembristi in the Ukraine?
To start with, it was 1825, not 1827. Then, there was no realistic chance for them to force Nicholas' abdication in Constantine's favor because Constantine already abdicated in Nicholas' favor

and in his letter tо Nicholas (written few days after the events) he characterized the rebels as scumbags ("swoloch") and the whole event as terrible and shameful. As an option, one can play scenario in which Nicholas abdicates in Michael's favor but probability of it is too close to zero. Actually, for the Decembrists the whole scenario with Constantine was just a cover-up allowing to raise at least some troops under a seemingly legitimate pretext (according to the historical anecdote soldiers thought that "Constitution"/"Konstitucia" is a name of Konstantin's wife). They did not have a single coherent plan of action but it seems that at least one of them involved a complete extermination of the imperial family.
"Mob" was not a military factor (looting, if the whole affair continued until darkness, was another issue). True, at some point Nicholas freaked out ("it is a miracle that we had not been shot") but when push came to shove he had more than 12,000 loyal troops (3,000 cavalry and 9,000 infantry) with 36 cannons against 3,000 rebels, enough for dealing with them AND with a mob. Plus at the city outskirts the additional 7,000 infantry and 22 cavalry squadrons (3,000) had been positioned as a reserve.
After all attempts to parlay failed (as a result, general Miloradovich while talking to the troops had been shot in the back) the artillery arrived and after few grapeshot salvos the rebels fled and an attempt to rearrange them on the ice of Neva had been dwarfed by using the cannonballs to break the ice (as Nappy did at Austerlitz). The 1st grapeshot salvo had been fired at the mob (which immediately fled) and the following into the rebels.
Rebellion on Ukraine also was on a pathetic side. The leaders (brothers Muraviev-Apostol) deceived the soldiers of the Chernigov Regiment and then allowed them to loot the drinking establishments on the route (in just one of them 360 buckets of vodka, оr 4,428 liters, had been stolen) and to rob the locals (numerous cases of stealing even from the poor peasants had been recorded) so, by the numerous accounts, most of the rebels had been drunk all the time. Needless to say that at the critical moment the leaders tried to flee but had been prevented by the soldiers: "you cook the meal, you'll eat it with us".