Voted 5. Don't know why people keep comparing it with the Marut specifically, but the other two Inevitables (Kolyarut and Zelekhut) only have fast healing 5 (and hp totals that are more in line with the Drenkari: 91 and 74 respectively, vs 112 hp for the Marut).
Plus, with the combination of immunity to weapon damage and a decent SR, I don't think it needs that much more fast healing.
By the way... Large constructs normally get a +30 hp, but since our base critter is Diminutive, it doesn't get those. It might be interesting to have the coalesced form be subject to weapons damage, but give it that +30 hp. That way, if the thing gets damaged too much, it *has* to stay in coalesced form to survive (and can no longer fly etc). Just an idea...