What if Sweden was able to beat the Russians at Poltava. In beginning of the battle in OTL things were looking good for Sweden, the first line of battlements was taken and the Russians pulled back. However, the Russians were allowed time to recover from the attack and then broke the Swedish line. So lets say that the success of the beginning of the battle snowballs into a retreat. Peter is beaten by a force less than an eighth his size. Could the Swedes still win the Great Northern War or was it to far gone by then? Could a win at Poltava inspire the 30,000 cossacks promised to Charles to revolt? Or was the war to far gone?
That's a pretty problematic interpretation of the battle IMO.
First things first: the russian army wasn't eight time the size of the swedish one, maybe two and half (20k vs 50k, or 30k vs 75k, depending on the author) still a solid advantage but nowhere as huge.
Second you drastically overstate the importance of the redoubts. If the redoubts had been held it would have been nice for the russians but that was never needed for the russian plan to work. The redoubt where mainly intempted to slow down the swedish advance, to make them more tired they actually where before they came into contact with the main part of the russian forces and fortifications and to expose them to the fire of said forces while they couldn't reply since they where busy attacking the redoubts. In fact, the original swedish plan was to bypass the reboudts during the night and attack the fort directly at dawn, witch would have demanded impecable coordination as well as allot of luck to work.
Third, the hope of having the cosacks defect was long gone by that point. That ship had sailed when the russians figured out the hetman Ivan Mazzepa was in contact with Charles and deposed him before the swedes got to Ukraine.
Basically, the russians had all the cards at Poltava: they had a commanding numerical superiority, they had fortifications the swedes had to assault and the old advantage in quality the Caroleans used to have was gone as the russian forces where done with their modernisation and the swedes where exhausted, ill and hungry by that point, a shadow of their former selves.
Even if the swedes had managed to pull it out despite all that they would still have taken important looses and you probably still would have other russians forces around and able to destroy them latter on while they would still be isolated. Basically, the swedes win because victory at Poltava scenario is a bit like the Napoléon win because victory at Waterloo one. It was already too late for that. The only way to save Sweden at that point is probably to get the Ottomans involved.
A Swedish victory could involve the death of peter, Sweden could still strike back, and with ottoman support Alexei might sign peace after another defeat
Charles should never have gone South in the first place. A more favorable POD would have been on his abortive thrust from Mogador to Smolensk, where he had to choose between marching North to Swedish Estonia or South to Ukraine. If he had gone North to St. Petersburg Tsar Peter would have had no choice but to face the full might of the Swedish army or risk losing the city he had staked all of his hopes and prestige on.
Charles should never have gone South in the first place. A more favorable POD would have been on his abortive thrust from Mogador to Smolensk, where he had to choose between marching North to Swedish Estonia or South to Ukraine. If he had gone North to St. Petersburg Tsar Peter would have had no choice but to face the full might of the Swedish army or risk losing the city he had staked all of his hopes and prestige on.
Oh, my friend, you have no idea how long I've been waiting to use these images;
So this is the area of St. Petersburg under Swedish Control in 1698, just before the Great Northern War
This is the area in 1704, one year after the capture of the area. By 1711 the original fortifications at Nyenskans had been destroyed by the Russians.
The only fortifications on-site during the Great Northern war - to my knowledge - were the main Peter and Paul Fortress, which initially consisted of only earth and timber fortifications (reconstruction with masonry began in 1704 and was completed around 1720), and the 'kronshlot' which was a small wooden tower and artillery redoubt seen in the above map at the far right, and in contemporary detail below.
I don't think either of these fortifications would have withstood an assault by the main Swedish army, especially not at the level of strength and experience it was at before the ill-fated expedition into the Ukraine.
In addition to this, and as I said above, Tsar Peters entire legacy and all of his hopes and dreams and authority had been invested in making St. Petersburg Russia's 'Window the West.' IIRC he had banned the construction of all stone buildings outside St. Petersburg, forcibly transporting native masons and expending large sums of money importing foreign ones to facilitate it's construction.
Tsar Peter was terribly unpopular to the native nobility for his modernizing ways, and if the Swedes had captured or destroyed his baby - I.E. St Petersburg - I think he would have found himself the Darius to Charles' 'Alexander.' By which I mean he would have lost face and been assassinated by his own lords.