really need help with buttons

[LunaMoo]

If activating dev menu by key crashes, then it's a bug, however you don't want to use "dev menu" in the first place, it's completely useless for normal users trying to just play games, honestly not even all that useful for users reporting issues since most issues are graphics related and require use of GE debugger.

What you want to map is "Pause", I think by default ppsspp uses L3 and HOME(the big center button often looking like logo on some xinput gamepads).

As for pressing a key "after" another key - nothing like that is implemented in ppsspp, if the game doesn't provide such functionality natively you would have to use cheats or any external programs designed to create such macros, for example GlovePIE, autohotkey and probably tons of other ones.