I could see Igni working as a god; definitely a great starting point, Velikolepni, for a god that's kind of the thematic opposite of Dithmenos.
I am not sure about making his abilities invocations-dependent though. I think Igni's divine ring of flames should scale with piety, and should be an "activate, deactivate" ability—i.e., it is like a form that doesn't ever time out. However, it takes a turn or two to extinguish after deactivation, during which time scrolls are still blocked. Rather than taking an amount of health up front as a cost (which steps on Makhleb's territory), how about some deduction to max HP while active? I think that would make it more differentiated, interesting, and unique. Also, it would remove any annoyance of having to "recast" it if you want to leave it on for extended periods.
It might make sense for the "fire form" that igni provides to lower your stealth dramatically or, perhaps better, continuously create noise. There's more of a trade-off when using it, then.
My one worry would be bad interaction with water, creating a bad interface problem. This could be handled in different ways; Igni's "divine fire" could be special-cased such that it does not cause steam clouds. That might be best. Alternately, you could be blocked from using the divine fire ring while you are in or adjacent to water; problem there is, it would be blocked in Swamp (that's fine) but then in Shoals it would be sometimes blocked, sometimes not, and dealing with that could be annoying.
The effects that "fire form" provides can scale up with piety, if you'd like. It would be neat if Igni, at higher piety levels, boosted fire-brand damage from weapons and gave a scaling spell power enhancer (equal to one enhancer effect (e.g., staff of fire) at max piety) to fire spells. You could also do things like, rather than have freezing brand weapons be simply unusable no matter what, make it so that (p)raying over non-artefact, freezing-branded weapons changed them to fire brand. (At the same piety cost as the "rekindle the flames" ability.) So freezing-brand weapons can't be used, but you get the ability to permanently re-brand them later on.
Anyway, overall I think something along these lines would give powerful abilities with wide applicability to different play styles, with clear advantages but also clear drawbacks, both to worship and to using the abilities themselves (there's some trade off).
It is a small thing, but maybe also have Igni give rSticky Flame upon joining, or from first star of piety onward?