I argue PeIg cannot create ice if only one form can create ice. This would be like using Perdo Corpus to cure a disease or remove a tumour. It may make sense to the modern mind, however it runs contrary to the setting and much of the game.
I also refer to Classical elements often and insert effects that seem to be missing in Aquam, Auram, and Terrem which are in Ignem. I find Ignem as written to be problematic in that it seems to be the special form which deviates a bit from Aristotelian physics and inconsistent with the rules other forms need to obey.
For instance PeIg can chill a person draining a fatigue without a Corpus req. Even though heat is a quality of the body, one’s inner fire may be Ignem, however the target being affected is Corpus.
Contrast this with PeAq used to dehydrate someone, which has a Corpus requirement based upon the very notion above.
Perdo Ignem to make ice I find problematic also because Perdo cannot sharpen a sword, it makes a thing worse. While ruining something’s heat source will make it get cold, if you’re making ice, heat is a quality of the water and is Aquam. And Ice forming a solid instead of a slush is a more perfect form of Aquam, Creo. Perdo also destroys all of a quality being destroyed, not part. So Perdo Ignem to destroy a fire’s ability to burn does so wholely, one cannot partially destroy its ability to burn such the fire will not burn cloth, but burn other things.
The mistake (in the RAW) I think is in the idea that Ignem governs Heat and Light as well as Fire. I don’t think it should, at least not uniquely. Hot is a quality of Fire. Hot is a quality of Air. Cold is a quality of Aquam and Terrem. Heat is a quality of other forms, it shouldn’t be a form itself.
Instead of trying to fix things by removing base effects from Ignem or adding requirements to existing spells, I find it better to simply add effects to other forms and remove the requirements where it makes sense.
Also Ignem is just too good and I find it upsets game balance.