I can not comment on the other bug, so I'm opening this one, I hope this is OK.
Building from today's hg repository, I indeed get ligated Lam-Alef, thanks for the fix. But there are some few annoyances:
* there is a gap after the Lam-Alef, it gives a false impression there is a space after it.
* when typing, the character typed after the Lam-Alef doesn't appear immediately until another character is typed.
* backspacing the Alef of the Lam-Alef ligature gives no visual feedback and it looks like nothing happened unless followed by another backspace which will now remove the whole ligature.