Chad,
Do you think fast flight helps eliminate handshock /vibration or not?
Definately. You still need proper tuning, nock fit, brace height, etc. but it sure helps. I look at it like this: stretch a rubber band tight, and a piece of twine, and have someone pluck them. The rubber band is much more elastic and will vibrate a lot more--dacron is much more elastic than FF.
Also, does Dacron ever stop stretching or does it just stretch less over time?
Depends. If you use enough strands and/or the temps don't get too high, it will stretch a lot less and may settle in completely.
On a heavier draw weight, and/or lower strand count, and/or in higher temps, it stretches a lot more. B-55 does seem to stretch less over time than B-500 or B-50. I can stretch a dacron string several inches on my jig--some of it contracts when the tension is removed.
I pre-stretch all my strings on a jig under around 300# of tension...but all the materials have some degree of elasticity.