Hellrifles, I can do; and I just did.
Eh, it's always struck me as something of a pointless limitation - the loss of grenades/sweeping advance seems like enough of a secondary balancer beyond increased points costs. Regardless of that, perhaps let Termie-armoured Inq.s purchase a Hellrifle so they can get their shooty-radical on?
Easy fix to avoid a bit of rules/fluff wonkiness.
Also, Sioban's Hammer seems a little odd now Daemonhammers are Force Weapons in addition to being Thunder Hammers. Perhaps change it to a Thunder Hammer with Daemonbane? (Daemonbane being the special rule granting Nemesis Force Weapons their anti-daemon qualities.)
I would like some sort of upgrade option for the turret to stay true to the theme, but I'm concerned that going up an Executioner turret might be a bit too much. It's a lot of dakka.
Like the look of the Bahamut; maybe have the Executioner Plasma Cannon available as an option to replace the Twin-linked, similar to the Tiamat's Melta Cannon upgrade?