First of all, I'd like to expand on my two sentence response since I fell I could have been a lot clearer. Although the INC had been formed in 1885 and had some some success in galvanising popular support after the 1905 partition of Bengal, it had spent the pre-WWI years as a shell of its former self because of a long drawn civil war between its moderates and extremists in the late 1900s. This ended in the expulsion of the extremists from the party and the British jailing the most prominent extremist leaders. Eventually the INC ended up welcoming back its expelled extremists during the Congress at Lucknow in 1916. The unified Congress then made an alliance with the Muslim League (on Jinnah's prodding) during the same Lucknow Congress. Although the movement was technically united after the Lucknow Pact, the underlying tension between the moderates and the Muslim League who wanted more Indian participation in government through dialogue with the British (with the Muslims wanting guaranteed representation) and the extremists who wanted complete independence by revolution was still present.
What I mean by all this is that coming out of WWI the Indian independence movement was at a cross-roads with no real way of predicting what the exact tenor of its future activities would be.
Although the British didn't really consider at any point giving India dominion status they did manage to mollify a significant number of the moderates by having Montague (the Sec. of State for India) call for more Indian participation in the administration. All that goodwill evaporated with the passage of the Rowlatt Act which severely clamped down on civil liberties and the ensuing Jallianwallah Bagh massacre . It was at this point that Gandhi really came to prominence when he used his concept of satyagraha to inspire millions to march in his Non-Co-operation movement. It was this movement and the resulting infusion of new members into the INC that fundamentally changed it from a party of the British educated intelligentsia to a party with members from every class and region in India.
Without Gandhi, this transformation is far from assured. IMO without the unifying figure of Gandhi and his large cadre of followers to enforce party discipline, the old elites of the Congress are bound to splinter again like they did in Surat in 1907. In the ensuing political vacuum, the only force that I see having the sheer propaganda power and organisation to succeed are the Comintern supported Socialists. And hence my scenario of a violent struggle with plenty of bomb throwing and assassination. Assuming this leads to high profile trials and execution much like OTL's Baghat Singh or the INA officer's Red Fort trials, the loyalty of the Indian Army and the level of success of the leftist rebels will be the deciding factor in how and when independence is given
To respond to some specific points you made
Firstly, before the Non-Co-operation movement I sincerely doubt that the INC would have been identified as anything other than a fractious group of London educated intellectuals with little to no base in rural India. Secondly, although Gandhi's infusion of spirituality into the Congress definitely put off the then still secular Jinnah (he did eat pork and drink alcohol after all), I suspect the underlying difference between the Congress and the Muslim League i.e the INC was moving towards full independence while the League wanted to work with the British; is what pushed him towards leaving the INC.
The movement's mass appeal pre 1919 was a bit sporadic. Sure it had achieved some popularity after the 1905 partition of Bengal but the ensuing struggle between the moderates and extremists badly damaged its standing in the national consciousness. It was the Jalianwallah Bagh and Gandhi's satyagragha that really turned it into a transcendental national movement.
Also Nehru hadn't always been a populist. According to what I have read what turned him from an intellectual elite (like his father Motilal) to a populist was Gandhi pushing him to tour rural India to give speeches to thousands of impoverished peasants.
Well here we'll have to agree to disagree. I admit that I may have exaggerated what were rather marginal trends IOTL but in my defence Gandhi's contribution to what I consider the rebirth of the Indian Independence movement after WWI is so fundamental that if you remove him all you are left with are marginal movements and tends. After all there is a reason us Indians refer to Gandhi as the father of our nation