Yes the engine can "catch" before 6th blade, how ever: if the outside temp. is realy cold you are never able to fire that merlin up like in 15°c or higher on the 6th blade. If you have near perfect condition's (outside temp.) you are never going to have issues to fire it up.
The Merlin is carbureted (like u mentioned) and its normal that it does not allways fire up on the exact same stroke everytime.
And the perfect example for a perfect condition startup is in this video
Some Airplane engines did have a magneto that would throw a spark as long as the starter was pulled. Idk if it is the case on the P51d.