The incentive to doing Hard and especially Nightmare is that otherwise the game is too easy and you just end up with a button masher. Combat becomes a lot more interesting when you need to plan things.
Two mages is fine as mages are always powerful and can fulfill different roles. Dorian is a Necromancer so you'll want to specialize in either Rift Mage or Knight Enchanter. Rift Mage if you want to gather things and nuke the shit out of them with unlimited MP, Knight Enchanter if you never want to die and you'd prefer swinging a Fade sword around to blasting things with a staff.
Dual-wield rogue can be fun, but the hit detection on daggers and moves can make it frustrating. It's also much squishier than any other class/weapon combo since it needs to be on the frontlines and doesn't have Knight Enchanter's unlimited Barrier protection.
You can have 2 Knight Enchanters. Combined with your tank you'll have a fairly unkillable party but it works out to about as you say. YMMV on whether that will be fun.
I got the recruits thing on my first playthrough but I did need to recruit more than 10.
Here's a dual-wield rogue guide:
http://www.reddit.com/r/dragonage/comments/2nm8bc/dragon_age_inquisition_class_build_dual_dagger/
For Tempest specialization upgraded Flask of Fire will be king, and also grab the passive for damage +15%.