That sounds like AwesomeWM might be a good fit for you. A lot of the 'window manager' is essentially Lua, which doubles as the configuration language. If, upon pressing the F12 key, you want your windows to perform a choreographed dance before organizing themselves into neat rows ordered by name, it is the window manager for you. Since all the interesting parts are written in Lua, you can easily modify it to look and behave like anything reasonable - and most unreasonable things - you can come up with.

Alt-IJKL, when remapped at a low level like that, should behave exactly like arrow keys and combo with Ctrl/Shift like you would expect. You don't have to do anything extra.