The question comes up a lot, usually people suggest that either Zinoviev or Kamenev would be likely individuals to take over as leaders of the party. Ultimately you've got to ask firstly how Stalin accumulated power and secondly in what conditions did this occur.
In 1920 Stalin was already the People's Commissar of Nationalities and People's Commissar of the Workers' and Peasants' Inspectorate in addition to being a member of the Party's Central Committee he was then made the head of the Uchraspred (which can be translated as the Accounts and Assignments Section). The Uchraspred was conceived to help organise the mobilization, transfers and appointments of members of the party and already in 1920 people were complaining about the situation of the transfers of party members as during the Ninth Congress Yakovlev said "The Ukraine has become a place of exile. Comrades who are unpopular for whatever reason in Moscow are deported there" and Yurenev made similar complaints. Now Stalin wasn't the only party member to accumulate several important positions at once, there was a general shortage of competent and dedicated individuals, but this position in particular, the Uchraspred, was vital for Stalin's accumulation of power.
In 1922 the Orgraspred, which can be translated as Organization-Assignments Section, was created and put under the purview of Lazar Kaganovich, an individual who was well known to be a close associate of Stalin. It worked in a similar fashion to the Uchraspred, allowing promotions and appointments of party members to key positions. Tens of thousands of appointments to positions of power were made through these two organs and they were eventually amalgamated in 1924. This accumulation of positions essentially allowed Stalin to appoint his allies to key positions throughout the nation and transfer political opponents away from the centres of power in Moscow. For example, the trade unionists Riaznov and Tomsky worked to make the trade union congress adopt a position contrary to the centralization of power and as a consequence Tomsky was transferred to Turkestan and Riaznov to a position abroad.
People consistently complained about Stalin's accumulation of authority in each congress until, ultimately, the people attending the congresses were mainly supporters and allies of Stalin so the voices of dissent were drowned out. At the Eleventh Party Congress of 1922 Preobrazhensky and others compained, he summed it up by asking "Is it conceivable that one individual is capable of measuring up to the work required by two commissariats, and on top of that the Politbureau, the Orgbureau and ten other commissions of the Central Committee?" but Stalin's power was justified essentially due to a lack of personnel and because of the Civil War.
It's true that the Party was lacking in experienced and capable members. Some, like Sverdlov, died in the famines and epidemics of the period (and generally being overworked) and some like Uritsky and Volodarsky were assassinated by the enemies of the Bolsheviks, Uritsky by the right-wing Cadets and Volodarsky by the Social Revolutionaries who tried to assassinate Lenin and Trotsky. Also the internal disputes within the Party led to some avoiding taking positions of authority out of protest such as Oppokov and others who refused to take up positions due to the Brest-Litovsk treaty despite the pleas of Lenin that they were needed.
In a way the Bolsheviks needed bureaucrats and organisation of this sort, basically to grease the gears of the machine, the fledgling state that was threatened on all sides by imperialism from outside and counter-revolution within. I certainly don't want to devolve into Great Man theory but the question has to be asked, if someone other than Stalin had been assigned to these organisational, bureaucratic posts would they have similarly accumulated personal power? And who, other than Stalin, would have used these posts to shore up their own power whilst maintaining a coherency of the state machinery to allow the soviet power to overcome the Civil War and emerge intact?
Sverdlov, if he'd not died, would have been ideal for some sort of similar position as he was well known as a key organiser. Potentially Alexei Rykov could have been given a similar position and used it as a route to power. Perhaps Joffe if he'd remained well-liked enough to be re-elected to the Central Committee instead of shunted off into diplomatic work? There's plenty of options, I feel.
I would talk about your other points but I feel I've already posted a really long post so don't want to ramble too much but it really depends on which individuals end up in which positions and how they use those positions when faced with the challenges the Soviet Union came up against. The Platform of the Workers' Opposition and the Platform of the Left Opposition lay out wildly differing policies to the policies of the Party under the control of Stalin, particularly regarding industrialisation, so it's not too ridiculous to suggest that things could have been very different if Stalin hadn't accumulated power. The lynch-pin, if you want the Soviet Union to remain a multi-party state with some semblance of internal democracy, is probably the Treaty of Brest-Litovsk because after that the Social-Revolutionaries took up arms within their terrorist cells and began targeting Bolshevik leaders which encouraged the Bolsheviks to adopt the positions of Red Terror.
One day I'll write a timeline about all this...