1. How to compose Fn lock if it is on the same button?
Sticky keys. Push Fn once, and the next key you press is the Fn version. Hit Fn a second time (ie, twice in a row) and you're in Fn lock, in which all key presses produce their Fn equivalent. Works for me.
Personally, I'd argue that shift can work the same way: hit shift twice in a row to achieve caps lock. That frees up Fn-Shift for AltGr so it doesn't have to be on the J in the middle of the board.