I had built my AR with a YHM Diamond Free Float Handguard and an A3 Flattop upper.  I recently purchased the Rock River Arm scope mount (seen here) for my rifle scope.  Problem is, I'd like to have flip up backup sights on the upper rail as well and the mount takes up my entire upper rail.  So here's my questions:

1) Is it a good idea to move the mount forward so that the forward attach point is on my handguard.
2) I've tried to do (1) but can't because the gap between my upper and my handguard throws the handguard off... is this supposed to happen or do I have the handguard installed improperly.  I currently have the handguard threaded as far down the barrel nut so that it nearly touches the upper.

If (1) is not a good idea, I guess I'm SOL and will either have to pick what I want on my weapon or I'll have to get a different scope mount that will allow both...

You can adjust the handguard so the rails line up as long as it isn't off too far.

Loosen the hg lockring and set screws if applicable. Turn out the hg until you scope mount lines up and can be fastened on, bridging the upper and hg. tighten the scope mount in place, then the lockring and hg set screws.

˄˄this˄˄
also they make some cantilevered mounts for this. More so to have a red dot and magnifier both on the receiver but i think it would suit your purposes as well.