1) When I first started crewing the H20, we had a roller furling jib also. The only way to get good shape on the jib was to tighten the jib downhaul until the forestay was visibly unloaded. Even with the current non-roller furling jib we tension to about this level and haven't had any problems. If you are having trouble with your jib luff flapping while going to weather, you are not footing properly. Unless it's really your leach that's flapping, then you need to move your jib cars forward. If you have raked the mast back, this is a common problem and the solution is to move the jib cars forward of the mast rotation cleat. I like to verify the jib cars position by watching my jib tell tales during a tack and adjust until they break at the same time.
2) The downhaul tension is essentially adjusted for the conditions. Tight to de-power, loose to power. Some skippers like a little more tension than others, but you essentially want the wrinkles out of the main when going upwind and loose when going down. And yes, the comptip will bend if you do max downhaul. This is, in my opinion, one of the benefits of the comptip mast, allowing for easier de-powering during heavy winds and/or gusts. If your crew is watching, they can de-power the main during puffs and help you maintain speed.