Darksmith: my crossbow I got is a low grade one, and with practice I split arrows now on a regular basis if I shoot at the same spot twice (though with my low power xbow my max range I practice is 35 to 40 yards)..you'll get it, and you got the right attitude as your havin fun.. my suggestions to how you described your probs:
-hell with un-cocking the bow, just shoot it into the target
-if your scope is maxed out on height at 15 yards, sounds to me like it ain't mounted right, but I could definitely be wrong
-with the draw string: I make sure every time that when the string is wrapped around the backside of the action/scope mount and the hooks are on either side of the rail, that my string is consistently placed the same way every time, and that the hooks as I draw back are both tightly against either side of the rail through the whole pull..when you are consistent on it you will find your grove to keep the drawstring out of the way of the cocking mechanism..

Also, after every time you shoot, it's good practice to wax your string and use the rail-lube, I'm sure you've already been advised of this..