you put it on the short string as soon as you can.
When you floor tiller it, and can displace the handle about 4" easily and have some even bend in the limbs, you shorten the string to give it about a 4" brace height.
THEN the real tillering starts.
As for the set, maybe you cranked it too hard too soon? Can you show pics of the thing from various sides?
The heat gun is your friend. With a piece like that, without a single knot, i would be experimenting like mad, and at the very least cook it into reflex. It's such a long bow: you could thin the handle, and make it a setback handle bow, which counteracts set and gives high early draw weight. I think.