OK, so you'd said that for continuity we'd say that Gabriel could apply Fell Frighten to his castings of Fireball. However, looking over things again with fresh eyes, I have an even better solution: Gabriel gets that ability from a magic item, anyway (Goggles of the Golden Sun, part of the Rainment of the Four set). So let's just say that his goggles are actually a bit different from the typical ones, and that instead of just casting Fireball his goggles cast Fell Frighten Fireball. Simple, easy, done.
Dumathoin could still be tinkering with his latest Hand, behind the scenes, if you like. Not a problem.
His set of items might even be modified to allow a warlock/invocation user to use them (typically they're for spellcasters, but I guess UMD might allow a warlock to "trick" them into accepting invocations instead of spell levels to work).
I’ve felt like the systems should be interchangeable, so… sure.
And any thoughts on the contacts (from anyone, DMs or otherwise)?
You’ve mostly focused on Toril, as a world. One more place there to find a victim: How about Thay? A Red Wizard just below council level?
Beyond Toril, there are lots of places:
Inner Planes:
Highly placed Elementals across the Inner Planes.
An official in the City of Brass.
Outer Planes:
The Outer Planes are more of a problem, because the senses of more creatures there might pick up on the psychic damage a Mark represents, so keeping a low profile is harder.
You may want to stay away from the Upper Planes because of this.
On the other hand, the Lower Planes are full of creatures with psychic damage, so what you inflict could easily go unnoticed against the background noise. You already have various possibilities you’ve identified yourself, on both sides of the Blood War and in the middle.
Also, you could expand your influence in Sigil:
Someone inside each of the key factions in Sigil.
Voidjammer Lines: See Upper Planes. Even worse, the level of paranoia is permanently extremely high; everyone who comes on board is first checked thoroughly with a battery of tests before they enter. So is the crew, on a periodic basis.
Wildspace:
There are a lot of places ripe for infiltration, as we’ve already seen in this game. Some are easier than others, while some have a high likelihood of detection.
Illithid Empire: Probably not the best idea. Psychic damage would be apparent to too many observers.
Bral: Wide open. Easy pickings.
Elven Armada: Rapid detection is a certainty; you’ve probably found out that all personnel undergo rigorous daily testing for all kinds of things, by magic, psionics, and other means. You’ve already found out how cautious and thorough Star herself is; they take their cue from her.
Elven Empire: Wide open. Easy pickings. It’s a loose confederation of worlds, so it’s much like any Prime.
Rienhart Industries: Largest shipping and courier service in private Spelljamming. It has its own military to defend its ships. Not as difficult to crack as the Elven Armada, though. A LE organization.
That’s a good starter list