The angle is checked whenever you touch R_Control. From looking at the code, the ship is meant to move in a certain way when beyond the Y bottom boundary as seen in event 140. This works for when he uses L_Control, as seen in event 149. If you set up triggers in the events you can see it gets all the way down to 140 but then he can't move right. He mentions this in OP :
"But to make this even simpler, because our founder appears to be really overburdened, what appears not to be working is in Line 140.
Basically make ship go down to lower boundary and once at lower screen boundary it won't go right at the default angle of 270, even though it should."
As I mentioned earlier the condition for 140 is never satisfied, but everything up until that point is working fine. Y < Player Y Bottom Boundary is a separate check to Y > Player Y Bottom Boundary. I agree that the controls are ridiculous though and the investigation took a while, Ashley would've been wasting time.