Shooting a longbow is a bit different than shooting a recurve... in most cases. As a general rule, the longbow has a much lower and less contoured grip. This will promote more hand contact with the grip. This isn't wrong. Most longbows are designed to be shot with this hand contact. Oftentimes, the limbs are tillered and timed to account for this pressure. You can try shooting them like a recurve with the higher wrist postition but this may make the bow want to move around more.
Try this. Hold the bow out in front of you. Balance it on one finger until you find the spot in the grip where the bow balances evenly. This is the bows center. Most longbows will shoot best when the pressure is in this spot. Some bows balance very low in the grip and other much higher. It's all in the design of the bow. If your bows center is low and you try shooting it with the pressure high it will want to fight you.
Pertaing to your original question, the lifeline is general starting point. You may have to find what works best for you. One thing alot of longbow shooters have trouble with is allowing the wrist to break inwards. I would suspect that the wrist is breaking in towards the riser on the shot. I've always tried to maintain a straight line from the back of my hand into my forearm. This will allow you to get a good hand position on the bow and maintain clearance from the string.
Your left hits could stem from a variety of things. If you are breaking the wrist inwards and inducing torque this could be the culprit. The other things that could be sending arrows left (assuming you are right handed) are overspined arrows and being unnaccustomed to the sight window. Many lonbows aren't cut past center. This generally requires a softer spined arrow. Also, it moves the arrow further from center, compared to a recurve, and may cause you to percieve the arrow further away. Sometimes it takes a little time to adjust to this.
One last thing. You don't mention if you shoot a dacron or modern, non-stretch, FF string. If you are using dacron, and your bow is approved for FF, then you might want to try that as well. Dacron stretches more and further during the shot. This can definitely promote a wrist slap even if everthing is correct. I've owned some longbows that were less than pleasurable with dacron but were gems with FF strings.
Hope some of this helps.
Chris