What you asking for is exactly either roaring opportunist + jorvulds guidance or zens redress + martial knowledge builds
Got grouped with a healer the other day that seemed to have nothing but DPS skills slotted, but was still consistently providing SPC buff to us somehow. When we landed in our dungeon they said something like, "Stay close to me and we stomp this quick" with no further explanation and off we all went.
Observing a little closer, they looked to be wearing SPC + Almalexia's Mercy which meant they could just rely on the constant auto-healing to keep everyone alive (as long as we were close enough) and buffed with Major Courage. Plus they had the Symphony set on so we all got a nice little recovery boost as well. All without actively doing anything to heal us, besides maybe some intentional positioning changes. They got to play the dungeon like a DPS, but they still fulfilled the role of healer really well... even though they kind of did that part on "auto-pilot". It was honestly kind of impressive. IIRC, the heal was about 2K per second, at all times.
It seemed like a pretty neat build that I'm definitely going to try after seeing it in action (we were in a vet Mazzatun, fwiw). Sounds like the kind of thing you're looking for, OP. Outside of an emergency burst heal (which if my example dude had slotted, I never once saw them use), this build might be perfect for your goal.
The only glaring difficulty I could see with this build is situations where you have to be separated from your tank for any significant time or distance. (Or if your randoms stray or stand in stupid a lot... or if everyone is running Pale Order -- but that's a problem with any build, so kind of moot.) It didn't seem to be an issue in our group. I have no idea what the tank was up to, or if the healer was pre-grouped with any of the other randoms I got thrown in with; it was just me and 3 strangers.
For your questions:
1 - SPC is your damage, Almalexia is your "utility"/healing, and Symphony is more utility (but only for group, not you)
2 - They were running double Destro staves, and I never saw any healing skills besides Purifying Light (if you wanna count that as one)
3 - Support from Symphony was nice, but I personally could live without resource support from healers in dungeons; I gave up on relying on randoms for sustain, and I reckon most people in vet randoms are geared up similarly. Go with a damage set if you like, imo.
4 - As said above, our healer skipped "must have" skills like Combat Prayer or any burst heal as far as I could see. They were doing Ele Drain and throwing Shards (and Orbs, but the damage morph on both), plus their Ritual of Retribution gave that synergy. Synergies are obviously good. Purifying Light was a nice HoT + the Minor Sorcery is always welcome. They used that Nova morph that gives the super synergy as well. Up to you how many of those are "must-haves".