you can use add_image instead of add_artwork if you want to keep all the artwork assets within /images. For snap or wheels use absolute path with [Title] or [Name]:
local sys_snap = fe.add_image (D:/path/to/image/snaps/[Title].png, #,#,#,#)
local sys_wheel = fe.add_image (D:/path/to/image/wheels/[Name].png, #,#,#,#)
This won't show placeholder when artwork is missing.
For display screen go to AM > Settings > Displays > 'Displays Menu' Options > Select Menu Style/ Layout (select theme for Displays Menu)
then go to AM > Settings > General > Startup Mode (Change to Displays Menu)
Now if [Emulator] is @ in displaymenu romlist then if DisplaysMenu has 5 items say 1,2,3,4 & 5. Item 3 has 5 items say A,B,C,D & E. If you go from 3 > D then press back it should take you to 3. That is how its working at my end.
I just saw progets answer, if you mean back key then default is Escape and mstly ppl use A for select/enter and B for back in gamepads.