The way we had our forwards do it in inline is have one guy do the digging down low and have the other in the slot, high or low depending on the player and where the play was happening. I guess we kinda played it like a PK is on ice really, one guy forechecks hard, the other stays back a bit in case he needs to backcheck. Having a guy in the slot is mega-importante in inline 4-on-4 since a pass to the slot intercepted usually means a breakaway the other direction. I assume your rink is a tad smaller than regulation like the one I played on was.
Basic rule with D was that if one (or even two) jumped into the play, someone had to drop back to cover him/them. It helped to have a guy like me who didn't have much in the way of offensive skill.
We never really bothered with left side/right side up front, it just sort of worked itself out. I don't know if you can do that with guys who don't have much playing experience, but it seemed to work out for us okay. We were much more stringent when it came to sides on D though.
All due respect, don't do a diamond. The box is easy to teach and it allows for you to always have two men at the point on the attack and always covering the opposition's points when in the d-zone. The guy at the high point of the diamond better be damned good and NEVER fumble the puck/ball, or else you're screwed.