Despite the fact that the Russian Empire had no representative legislature until 1905, no unified executive government in any serious sense of the word until 1906, and nothing that we would recognize today as a judicial branch until 1864, there were certainly external forces leaning on the emperor to emancipate the serfs in 1861 which I'll talk about in greater detail below, but the fact of the matter is that no other person or body of power could have actually authorized such a thing as the emancipation of the serfs besides the emperor himself. Full stop.
Were there external forces pushing on the emperor to act? Sure. Could he have ignored them with impunity? Yep. Was there any check on that power to prevent him from doing so? Not even close. Would inaction have resulted in a government-changing uprising which preempted the Bolshevik Revolution of 1917? Unlikely.
Remember Alexis de Tocqueville's famous refrain from his 1856 ruminations on the French Revolution, The Old Regime and the Revolution:
The regime that a revolution destroys is almost always better than the one that immediately preceded it, and experience teaches that the most dangerous time for a bad government is usually when it begins to reform.
Does Aleksandr II deserve 100% of the credit for freeing the serfs? Clearly not. Does he deserve a great deal of it? Yes.
Before launching into the description of those external forces, keep in mind, it's not as if Aleksandr II woke up one day and thought, 'you know, it's time to give up the fight and free the serfs.' Long before 1861, he-- and other Romanovs before him-- had been working to decrease the number of Russians who were considered property in the eyes of the law; after Pavel I (dec. 1801), they all realized serfdom had to be meaningfully reformed.
Consider Richard Pipes' analysis of Aleksandr II's pre-emancipation reforms:
Within the narrow limits open to it [...] the government did what it would to reduce the number of serfs and to improve their condition. Alexander forswore to hand out any more state or crown peasants to private persons. He also introduced procedures by which Russian landlords could carry out private emancipations, and authorized the liberation (without land) of the serfs belonging to the German barons in Livonia. The cumulative effect of these measures was gradually to reduce the proportion of serfs in the empire’s population from 45-50 per cent at the close of the eighteenth century, to 37.7 per cent in 1858. Serfdom was clearly on the wane. [1] (see pp. 163)
I guess moving from enslaving 50% of your population to only enslaving 40% of your population doesn't sound like a lot given 150+ years, but that is a reduction of 20% relatively, and given Aleksandr I was hardly a reformer in any serious sense of the word and Nikolai I wasn't a reformer even in a non-serious sense of the word, that only strengthens the argument that Aleksandr II really did drive the reforms that led to emancipation.
That all said though, the emperor's uncle (Aleksandr I) had introduced some really, really primitive reforms to the system in 1802, allowing a serf to be considered a person (thus meriting certain allowances). It's not much, of course, but it's a step in the right direction. Likewise, the emperor's father (Nikolai I), while lying on his deathbed, had made the young Aleksandr Romanov swear to him that he would work towards the emancipation of the serfs-- the only such demand he imposed upon his young son in the hours before his death. [2]
Also important to keep in mind, for every external force pushing on Aleksandr II to free the serfs, there were just as powerful counter-forces pushing on Aleksandr II not to do so. The landed classes and nobility of Russia understandably stood to lose quite a lot should their labor force just be up and freed one day. Indeed, the debate surrounding the freeing of an entire class of enslaved people would lead to the outbreak of a gruesome civil war on the other side of the ocean just 52 days after Aleksandr II proclaimed the emancipation of Russia's serfs on 19 February. One might even make the argument that due to the divisiveness of the issue, had the Russian Empire been ruled by anything other than a singular figure, it would have experienced a similar history-altering fissure.
The decision to proceed with emancipation, come what may, was taken very soon after the accession of Alexander II. It was carried out in the teeth of strong resistance of the landowning class and in disregard of formidable administrative obstacles. Scholars had once believed that the step was taken largely on economic grounds, namely as a result of a crisis in the serf economy. This belief, however, does not appear well grounded. There is no evidence that economic considerations were uppermost in the government’s mind when it took the decision to proceed with emancipation. [3] (see pp. 163)
Pipes says the driving external factors behind emancipation were political in nature shortly thereafter. In other words, Aleksandr II was clearly committed to emancipating the serfs at a personal level from the start of his reign but further than that the reasons he was able to actually pull it off, in the face of such significant opposition, were political in nature. With respect to the actual implementation of the decree, Aleksandr II certainly had less to do with the paperwork and clearing of the administrative obstacles Pipes mentions above-- that responsibility fell largely upon Nikolai Milyutin (who was likewise largely responsible for composing the legislature itself). It's occurred to me as I reread this answer that you might have been interested in the actual, physical process of emancipation-- if so, I'm happy to write more about Milyutin's own work as well because it's an interesting topic that saw the man dismissed from his advisory role in Aleksandr II's government the day after emancipation was announced.
Barring that for the time being though, the primary forces for the emancipation of serfdom were the following:
The primary forces against emancipation were:
Although it seems like a pretty open and shut case for the emancipation of the serfs, the nobility of Russia still wielded significant power (more so than elsewhere in Europe at least) well into the 19th century. Obviously, this is shown by the fact that there's a lot more to say about this story after emancipation took place because a great deal of these serfs would be more or less forced to keep working for little to no actual profit for the exact same people who had previously owned them, and the emancipation itself wasn't exactly the end all be all that we remember it as. Despite direct non-compliance, the Romanovs couldn't just persecute the nobles they relied on for legitimacy.