i would imagine any diagonal movement is just two cardinal movements together. also, from my understanding, while doing anything related to this glitch there is a heavy desync between the room you think you're in and the room you're actually in. for example, you can probably get into this nonexistent room with a different room's visuals on the screen 
visuals is probably not the right word. the room acts the same, but your location on the overall map is different. in that last video i posted, i went up in the bottom right room (red arrow) and ended up in the top left room (blue arrow) 
Yeah, with failed OOB attempts sometimes you end up in the wrong room, try to get back to the original room, and get warped to a completely different room when crossing a border, it's so weird
Just to unify information: when you warp, the "expected" screen is loaded, but your internal position on the grid of screens (confusingly called xpos and ypos in code) gets offset incorrectly. The latter determines the tileset/palette of the screen and, importantly, to which screens the edges of the screen will take you (which is why once you've done a regular transition, there is no more discrepancy). If the room that is attempted to load exists, it will load, even if that puts your internal screen grid position onto a hole in the map or outside the map bounds entirely. If it doesn't exist, you just die.
Also quite importantly: the boundaries of the entire map (ie. the edges of the rectangle containing all the existing screens, spanning 10 screen widths by 10 screen heights) are sort of "reinforced"; there is only one precise position value that will let you perform a warp if you are up against them, and you can't bounce off because that prevents screen transitions for that frame. That's why trying to do it at the top of the overworld might seem more difficult than in other places.
