Also, point of inquiry: what happens to immaterial creatures when they hit a Fire Shield?
In addition to what J said: if you're asking what
should happen, presumably the same as when a plague hits.
There is absolutely no logical reason why Rain of Fire should be the ONLY card/ability/spell that hits immaterial creatures when NOTHING else hits them.
I don't think anyone has argued for RoF to be the only card to hit, RoF is just the most obvious/popular AoE spell. Plague and Thunderstorm should presumably be considered the same.
In my opinion, the only thing that needs to be changed as far as the Immaterial status goes is the wording. Make it something along the lines of "is immune to worldly effects".
If it just prevents targeting, spells that don't (obviously) target will cause confusion as it is now, and probably even if the mechanics is changed.
On the other issues:
Bone walls stacking makes sense but will require some rebalancing.
Poison should be reflected. Miracle shouldn't, as you (the caster) are "inside" the shield. Should probably be extended to the point where no spell cast by you is reflected by your shield.
Cancelling an ability should make it as if you hadn't used it. It's not an issue of stupidity, it's usability.
Having all abilities available to mutants would make balancing impossible.