The Darkmark isn't really that useful as an outright combat power, at least not for another tier or two. Even then, the circumstances for its use as an offensive option are fairly narrow, though very cool if Garin wants to fuck over someone.
For the most part, the Darkmark as I've written it starts off as a really helpful tool that will allow Garin to coordinate with his Inquisition agents and other assets. Between the Dragoneye Rune, Status, and Telepathic Bond effects, he can locate them as needed, monitor their health and overall condition, and communicate with them at will. There are even quite a few 1st level spells he has access to that he could place on an agent in an emergency.
Garin has access to the
Assassin and very limited but effective
Shadowlord spell lists. That means that even at 1st tier, her could render an agent Invisible, grant them Darkvision, place Featherfall on them, etc.
Viserys is a Dragon and Lya is a self-mage Outsider, both of them with superhuman Constitution attributes. This necessitates the redefinition of quicky, at least as it applies to them.
I would like that, too, but a single Path ability shouldn't be able to do but so much. I think we've already packed as much into the basic Darkmark ability as is reasonable. Further functionality can be added at future tiers, maybe at 3rd and 6th tier, when new path options normally become available? Greater Darkmark at 3rd and True Darkmark at 6th?