Thanks for the tutorial link. It makes a lot of sense, so I shall add it into my good habits. It detects the collision much better.
After fiddling with the program a bit, I noticed a peculiar phenomenon. The Scroll To only seems to work if Unbound Scrolling is Yes. If set to No, it doesn't scroll at all. I'm also wondering if it has something to do with the Custom Movement. Perhaps the Scroll To behaviour is set at 32, but since mine is moving at 64, it's not playing nicely.
Edit: I tried setting the Actions; Scroll to Object, and Scroll to X/Y hoping the same functions to make the grid movement work would work on the Scrolling too. No luck. It also doesn't matter whether the Scroll To behaviour is added to the invisible player sprite, or the animation: it behaves the same with the slow scrolling.