The marshals mutinied because they thought they couldn't win. Napoleon had already abdicated before Marmont's treason, not after. I don't think the army would have followed Ney, they would have followed Napoleon and were prepared to do so when Napoleon abdicated.

And Net, et al, did not want the Bourbons back but wanted a regency for Napoleon II. Marmont's treason ruined that idea and made Marmont a pariah as well as a traitor. Again, see Caulaincourt's memoirs.

Marmont surrendered on April 5. Fontainebleau was agreed on and ratified between 11 and 13 April. I think if Ney and the other Marshals followed Marmont and surrendered, Napoleon would have been forced to abdicate. Whether they thought they couldn't win or win big enough to bring about a favourable treaty position, as opposed to revolting because Napoleon refused to see the same thing, is basically the same. Napoleon would have fought on if the Marshals, and Marmont had stayed loyal, the troops' loyalty were worthless without their commanders. Which is what Ney meant. And I'd not udnerestimate his popular pull on the rank and file. If Ney went Napoleon's prestige would have taken a big dent. Hence he called him in for 1815. He was living morale.
As to the expectations of the Marshals, it would be a vain hope that Napoleon II would receive a regent, but yet I agree the Bourbons did break the stipulations of the treaty while Napoleon was on Elba.