It's a good idea to carve the neck before giving radius to the fretboard... when you remove wood from the back of the neck, the remaining wood tends to stabilize according to the new shape, which means that the neck could get bent, including the front side... so you can loose the leveling. I usually wait about two weeks from carving to giving radius.
Sorry if it's too late for reply. ^^
There's no science behind that. A LP doesn't have any slot, but you can find it in a telecaster. In your case, I would make a slot, since there's no step between the 'fretboard' and the headstock...
I would make it cooler... 20-21 mm is the ideal thickness for a neck IMO (including the fretboard)... I usually give 15 mm to the neck + 5-6 mm to the fretboard. If the scarf joint is well done and the truss rod channel is the usual (about 9mm) should not be a problem. And you will not regret.