A number of things nag at me here.
First, we get the sense Decker lacks deep space experience, certainly compared to Kirk. What bugs me about this is why is an inexperienced officer being given command of one of Starfleet's best ships meant to go into the unknown? In "Where No Man Has Gone Before" we understand Kirk does have prior command experience with the reference to his "first command." So Starfleet didn't promote Pike and just hand the ship over to a neophyte.
Now we don't know enough about Decker's background so maybe he did have prior command experience. But I believe Scotty referenced an "untested" crew. Did that include Decker? If so then I go back to my initial point.
Whatever the case it does look like Decker is shaken to some degree when they encounter the Vger cloud. He has technical knowledge of the ship's systems, but he seems to lack intuitive instincts like Kirk has. Decker was inclined to play it safe, but he didn't seem to really get they didn't have a lot of time to play it safe.
I have no doubt Scotty would have eventually figured out their warp engine issues, but again they were pressed for time. Spock's arrival basically sped up the solution Scotty would have reached eventually. We have also seen Scott and Spock work together before so I doubt Scotty felt slighted by Spock's help, although he might have been taken aback by Spock's cold demeanour.