Is there any way the mission would've succeeded with Will Decker in command?
One of the jobs of a first officer is to present the Captain with option that the Captain might not have seen (Riker and the depressurization of the flight deck). Just because Decker pointed out the possibility of a phaser strike on the tractor beam emitter, does not mean he would have made the strike himself as Captain.
Similarly, another of the first officer's job is that he runs the ship, while the Captain runs the mission. So Decker's avocacy of more warp simulations prior to the first warp attempt was not a example of excessive caution, but him simply doing his job properly. Give the resulting worm hole effect, his advice to Kirk was correct.
Decker's position, his mind set, would have been different if he had been in the Captain's chair. The weight of the mission would have been on his shoulders, and he would have been the one making the command decisions. I believe he might still have delayed the first warp attempt, however the need to press on into the cloud after the Enterprise as attacked would have been Decker's priority, instead of Kirk's.
Don't forget, it was ultimately Decker's decision (and not Kirk's) to join with Illa and V'ger that accomplished the mission and saved Earth.