Yeah, the attack would technically be coming from you but it would be triggered by other players. I think that's the best way to handle it balance-wise. Having it scale off the target means you're bias toward DPS players, and that's probably not healthy for encouraging you to use your card effects on the best target. The added benefit of Time Dilation as a GCD action and allows you to always have a card effect in play so you're never in a bind where no one has card effects and Draw is on cooldown so you have to Malefic if you don't have time to heal. It's possibly you could let your cards fall off and find yourself in this situation, but it would just mean AST would be a bit more technical, which I think is fair considering this design would be about making them as passive as possible. There's the risk that a passive healer is a low-skill healer, but elements like this help make it so that you have to work to stay passively applying your flat potency buffs to allies.
It probably would be tricky, and like I said, it would probably get written out and then thrown to the wolves, figuratively speaking. There would be a lot of discussion points regarding this concept, and we would probably be prototyping it aggressively in private servers. Basically, though, what makes it dynamic is a bit about what we mentioned. Your best DPS comes from this healing buff that you can apply only to allies with card effects on them, so we ideally want to create a "rotation" of healing and shielding perhaps that allows the AST to keep their buff up, possibly on multiple allies at times like after Sleeve Draw and keeping up defensive buffs with Time Dilation. There would probably be moments where there's nothing you can do to keep this cycle going, which would be the best time to apply Combust, use GCD healing, and perform a cast or 2 of Malefic if nothing else is needed, but ideally this would be minimized as much as possible without making the build unforgiving for when you need to stop and GCD heal.
What about healing or mitigation tools that slow down the rate at which you actually receive damage so that your regens can keep up? You could have something that mitigates the damage you receive by, say, 90%... but the 90% that you don't take is instead spread out in the form of a DoT over 9 seconds, or so. These are all just random numbers, so adjustments can be made of course, but if the issue is that they can't burst heal... slowing down or dividing up burst damage is both a thematic and interesting solution.
That's a good point. Charges would make this version of Minor Arcana perfect. You both know when you're getting the Lady and can hold onto her for a few moments while you find the best time to use her. There are a lot of changes I don't expect to see regarding the healers that have been talked about, but this is one that I think isn't too unreasonable and makes a lot of sense. I really hope this shows up in 6.0