The way I see it, the POI changing on account of flexing is moot under 50 yards. I've got mine zeroed at 25 yards. Figure in the dark when the laser is applicable, you'd be shooting less than 25 yards...and even if you stretched a shooting scene out to 25-40 yards, the laser would still be within 10" of impact, which is fine for 'in the moment' shooting.
[img]http://www.mattdoughty.homestead.com/files/Hokie-AR.jpg[/img]
On the Knight's handguards, I don't think the flexing is too relevant. They're pretty rigid.
The handguard you have, if I'm thinking of the same one, should be fine as well. "FOBUS?" "M33?" I forget the name of them.
Regarding the sling. It's my opinion that a new sling is cheaper and easier than purchasing a special, unnecessary add on. A sling like this [url]http://www.bushmaster.com/shopping/slings/std-bp42-bk.asp[/url] is cheaper than most rails, adaptors, blocks, etc that you'll find. I have this sling and it's great. Let's the rifle hang BELOW the sling instead of OVER it. Makes sense ya know? Comes off easy too, so you don't have to keep it on there when you go to the range, or store it in a safe.
Hope this helps, good luck man. Nice job on the M6, you'll like it. Weighs next to nothing, has 80-90 lumens, laser, and an easy switch! I think the only deterrent is the price. But once you got it, you got it.[;]