Finally: Why do I see such a lack of arc nodes with him? I would think that having at least one would be important for him to stay safe and still cast his buff spells. Even the lists I've been creating seem to be lacking one.
So, to answer that, let me go through my preferred list with Tristan2 and the logic behind each piece. (Warning: This will be long, and possibly completely useless, but I'm bored and so here I am). Or you can go with the short answer: There's no room with what else I want in the list.
Tristan2 - Creator's Might
Hierophant (free)
Devout
Castigator x2
Templar x2
Indictor x2
Wrack x3 (free)
Vassal of Menoth (free)
Vassal Mechanic x2
min Choir x2
Hierophant - Hand of the Creator and Manifest Destiny are both expensive, you'll likely want to cast both on when possible, and Tristan is squishy and needs to camp as much as he can. Harmonious Exaltation allows for all that to work.
Devout - A 7th jack is needed to max out the feat, and Tristan is squishy. An argument could be made for a Vigilant, but I find having some actual offensive capability is useful late-game, and I prefer flat-out spell denial + shield guard to +4 DEF.
Castigator x2 - So all the heavies run in pods of 3, with one of each heavy per pod. This is why I have duplicates of everything. As for the Castigator, the list is very vulnerable to being bogged down by numbers. Castigator is the best, cheapest jack answer we have to swarms. It is there solely to provide combustion to clear a jamming infantry unit.
Indictor x2 - It has a shield, which is really nice when this list is NOT going to get to alpha. A blessed weapon gives the list the ability to go through buffs that it would otherwise struggle against, and the anti-magic bubble is just gravy. Note: If the anti-magic bubble gets bigger, and Indictor cheaper, I will be VERY happy. Did a CiD game with Durst and 2 Indictors, drove my opponent crazy with the bubbles.
Templar x2 - Has a shield, and shield-guard to prevent Tristan from being snipped out, or prevent annoying disrupt/KD shots against other targets.
Wracks / Vassal - More focus to allow jacks to run better and allow Tristan to camp.
Mechaniks - Have the points, and really want a way to fix these jacks, as most alphas tend to damage, but not completely scrap, one of the heavy jacks. These guys + Hand of the Creator can undo a lot of damage.
min Choir - since my jacks move in pods, they are often on completely separate sides, with Tristan in the middle. One choir has neither the bodies nor command range to cover all of them.
So then the question is, if I want a node, what do I cut?
I could cut the Devout + a Mechanik for a Revenger, but lose out of shield-guard + anti-magic, and experience shows if somebody can get a shot on Tristan, they absolutely will. Distance is not an acceptable defensive mechanic, as Manifest Destiny requires the jack to be within 14", so realistically Tristan will probably be at most 10" from the front line, easily in shooting range for a LOT.
I could cut a Templar for a Guardian, but then I have a less durable jack, making it more likely something dies when I get alpha'd, and also lose out on a shield guard.
I could cut an Indictor for a Guardian, but again I drop in ARM then. This is probably the best swap at the moment, though.
So none of those options appeal to me (and cutting a heavy for a Revenger + support seems terrible), so I don't use an arc node. Yes, Tristan could use his feat to, in theory (no jacks died), send 3-4 Cleansing Fires out for an assassination, but that seems like it'd be a "caught me once, never again" trick and overall unreliable. So I'd rather just spam ARM21 jacks, absorb the alpha, use Hand of the Creator/mechaniks to fix any damage, and strike back HARD.
TL;DR : Arc Node feels like it leads to an unreliable strategy, more comfortable spamming shield jacks, which leaves no points for a node.