Bungie said it was a mistake? I read plenty of their updates and never saw that dual wields were a mistake. Honestly 343 could re-engineer the sand box a bit and make it hyper functional.

The needler problem can be subverted by letting all needle based weapons combine together and keeping the classic needler, adding back the needle rifle, adding in a needle pistol that can be charge like a fusion rifle from destiny to fire a small needle burst with high spread slow bullets and fair range. The rifle and regular needler would take both hands the needle pistol would be dual wieldable.

As for the rest of the dual wielding you can simply try what halo 3 did and and leave dual wielding in but just add two hand weapons that do the same job, it seems redundant but having dual wield stuff in play creates a bit more havoc and technical play, that being said mauler should never return to halo in the dual wieldable variety.

To be quite honest I think H2A stumbled into a solution for this by offering up the suppressed SMG, it's pretty much an AR with a scope and it has higher fire rate than the normal SMG, it's quite similar to the H5 AR to be honest; you can suppress players, pop people out of scope and make kills at longer ranges but you still want something better the only difference being I can I have to ditch the scope to up my DPS and I have to ditch my DPS to throw grenades it makes dual wielding effective and technical while balancing it by keeping effective marksmanship and grenades a split second further away.

Halo 2 was very nuanced in higher level play, most top tier guys ran BR out 80% of the time only swapping for dual wields when the were in a pinch but those pinch moments made or broke games and that tension of being theoretically under prepared mad it interesting to watch.

I don't think dual wielding was a mistake in any way, truthfully I don't seem much of a point for the H5 SMG