Haha! Cool, glad it worked!
I wouldn’t say it was a noob question -maybe not the question an “expert” would ask though, haha- but just a question from someone who got lost and started to try things he isn’t very used to work with. It happens to me all the time too, haha.
In this case the casting, wasn’t the problem, really. The problem was caused by how you were using the array. In fact, the array is still a pretty good solution if done correctly. For example, using the current system you may get unexpected results if the character is overlapping more than one trigger at the same time, as we’re passing only the last actor the player overlaps, which may not always be the right one. So if you’re planning to place several walls next to each other or in any other way that could cause them to overlap, it’d actually be best to create an array of climb wall bp’s in your character bp, and instead of just setting the climbWall variable to self in your trigger, add a reference to self to that array. Then in your custom event use a for each loop and use some maths or tracing to get, for example, the distance between your character and each actor, or even better, check what actor the character is facing and based on that distance/facing direction get the “right” actor, get it’s arrow location and rotation, and eventually move the capsule.
Also you must “remove self” from the array in your end overlapp event.
It’s a bit more complex, but sort of more precise. But again, this is only useful if there are any chances that your character could overlap several triggers at the same time. Otherwise the current script will work just fine. We don’t really need features we aren’t going to use, do we? 
Cheers!