Hey was wondering what you guys thought about running 36 mana /28/28 to keep some RVR utility while being viable PVE until respecs are circulated around. Gets demezz, PoM 4, 2nd highest AE dot for dot kiting during lvling, and decent light to help base nuke variance. I will eventually be going full light for RVR, but using the free respec at 40 and going light will make 40-50 a drag for leveling. Is training 28 in light worth it to help out the base nuke as I will be running with a heat debuff train in RVR.
I am apprehensive to go full mana as I dont know how fast respecs will get out into circulation and how expensive they will be. Full mana spec is beyond useless in any RVR setup as the base light nuke will have such a high dmg variance.

In PvE, anything remotely useful is viable. You'll still be helpful to any group that hasn't a mentalist yet, just less so than a pure mana or mana/ment spec. I doubt any group looking for a mentalist would reject you, though. I know I wouldn't.

Talking RvR, it depends on what you consider "viable". The Mentalist's niche in optimized RvR groups is that of a blaster with some minor utility which you cannot fill without spec nukes. You wouldn't be useless, you'd bring yellow crack, which could be useful in dragged-out fights, but I would rather bring a second Eld or Bard, a third Druid, a Warden or another BM or Druid. Since these choices may not be available at all times, you might get the occassional random group though.

Edit: Since you said you'll be running a debuff train, which I oversaw initially, groupability doesn't seem to be an issue. Talking fixed groups, it's a suboptimal spec, but if you are playing with friends or a guild who don't mind, you guys can probably work around it. Leave the heat train aside for a second and shift your role towards support, with demezz, rupt and heals being the priority, in that order. If convenient, you can jump on the train and do some damage with base nukes. It's kind of like running a nature druid or battlewarden. IMO, your group would be better off helping you afford a respec, though.

TLDR: I'd go 46 mana, 28 ment and single respec mana into 45(46) light, 29(28) ment, rest mana later. Your spec is neither much worse for PvE nor much better for RvR (arguably worse, neither your HoT nor PoM is useless in RvR, IMO) than mana/ment, though. You wouldn't want to be speccing into light until 46 anyways.

Another option would be to drop mana to 34 and bump light to 31 which would allow you to charm the buff pet and some better pets more easily as many mobs would be questionable at the 28 light spec. Only thing that you would lose is ae dot which has little use in rvr. From a utility perspective having a pet that can interrupt sufficiently In addition maintaining yellow PoM and HoT. You can have quite a wide role in rvr until respeccing full light.

The tradeoff you suggest is most definitely worth it for RvR. If you're adamant about going tri spec and these are the alternatives, the latter one is better. That being said, In my opinion it's still far from being worth the group slot in RvR and mana/ment isn't much worse (arguably better) even for RvR. You'd want a respec ASAP either way.