Enforcer isn't legal.
And does the Defiant's siblings have dorsal mounts? Does any LC for that matter have dorsal mounts? The designers could still have kept the FP2 WBs and Str 2 torps but obviously they decided to change it to something closer to a Dauntless' prow weaponry, lowering it by 1 and giving it more flexible firing arcs. Otherwise it is a Dauntless' style prow. I was actually rather surprised they didn't give it Str 2 torps. Sure it would be a more powerful prow but then it would fix your issues about it not having enough equivalent WB firepower esp if pointed correctly.
Enforcer is varyingly legal. It's not in the official FAQ, but it's pretty rare to have someone say no to it, at least in my experience. As I said: increase LB. Defiant will be fine with 4lb, and this solves the problem of it being so broken as to not be used. A limit of 1 to 500 or 1 to 750 sounds good, and despite all the AC haters screaming, it won't be a second coming of the all AC IN fleet from 1.0 that they're all saying they see in their crystal balls and puffs of green smoke.
AS far as an Aboat option, sure, if it's going to be 2 lb, aboats are fine, as it's supposed to be a specialist ship like the rest of the voss.
As far as SC -1thawk +1 shield: No. It would absolutely cripple existing pure SM fleet lists against AC by nearly halving their LB, and they're weak enough against AC as is, unless you're also giving them all +2 turrets too. (And never be approved by GW for fluff reasons)