two things. First, in the quest I'm making, I made a custom subscreen (my first time doing so!). Problem is, I'll be able to cycle in one direction just fine, but after getting to a certain item, I won't be able to cycle the other direction.
Second, How do you make additional button items (L and R, Ex 1 and 2, etc.) Speaking of which, what exactly are the Ex buttons meant for?
Item cycling order is set by the how the directional arrow buttons advance through your subscreen while it's open. For example, if you have the boomerang equipped to the B button and press R, this will go to whatever item you'd get to by pressing Right from the boomerang on the active subscreen. If there's a problem with cycling to/from certain items, make sure your active subscreen is set up correctly.
Additional button items require scripting in 2.53 and below. The latest 2.55 build (a92 at the time of this post) supports equipping items to EX1/EX2 (called X/Y in the quest rule) but there are some bugs with this that are being ironed out.
EX1, 2, 3, and 4 are generally only used by scripts. There is a quest rule to make EX3 and EX4 cycle through A buttons (exactly like L and R cycle through B buttons) but that's the only way that they are used by default.