Making the war longer and bloodier isn't hard; ultimate success would require some systemic problems to have been corrected years before, I think.
There's no one among the rebel officers with training in logistics or strategy. Even when an Indian was promoted to Captain or Major, they weren't given the same training that white officers were - and during the rebellion that was glaringly obvious. Luck will take you so far, but systematically destroying the telegraph lines and railroads would have gone a LONG way towards handicapping the British, especially if done early, and in two years no one did so. Potentially, you don't need a great general, just an extremely enthusiastic vandal: Hobson and Nicholson stranded in the Punjab and lynched, no effective way to recruit the Pathans and Sikhs, the Gurkhas won't be enough by themselves so one of the other two Company Armies (Bombay or Madras) will have to be used and won't that be interesting...
I don't think redeploying the Regular Army is an option; it's true that there was public outrage over the Sepoys, but there was also tremendous backlash from the Crimean only a few years ago. I would expect riots in Britain if any meaningful number of Regulars was redeployed.
Early death of Zafar/Bahadur II might actually be a good way to go. If Mirza Khan (for example) had come out of the succession on top, the Delhi government might have reconstituted itself. A 79-year-old prevaricating poet is not the ideal choice for even a figurehead of revolution.
Easy to imagine the siege of Delhi getting broken at the 4-month mark. At that point, the rebels head down to Oudh/Lucknow and turn it into a meatgrinder.
Two core problems for the rebels: the jihadis make fewer demands for payment and supply and show more willingness to obey orders and maintain discipline than the typical soldiers do. A successful rebellion is going to have to rely on them, which will create problems in Lucknow and even bigger problems after success. The other is that they have to fight a two front war; the Company is capable of attacking from Bengal or from Bombay, and even if Punjab, Northwest Frontier and Kashmir become secure, finding troops to fully man both lines will be hard for the rebels (unless the Company calls up the Bombay Army and it mutinies on the scale of the Bengal one, but that's not certain...).