Cleon
Legend
Yeah, that's definitely the problem. I agree with keeping it simple. How about we have it reverse the certain spell descriptors as you indicate, changing energy type for damage as appropriate, as well as "description" (ie, mirrored sunburst looks like a blast of inky blackness) and positive-negative energy effects (in the sunburst example, perhaps it partly heals undead and fungi).
We shouldn't forget it also covers "reversible" spells.
Anyhow, I'll try roughing something out...
Mirror Spells: Kamerel mirror magic includes a special class of spells called mirror spells. A mirror spell is a spell that can be cast as one of two spells whose effects are the opposite of each other. For example, enlarge person/reduce person is a mirror spell that acts as either enlarge person or reduce person when cast.
A kamerel spellcaster can use one mirror spell at each spell level they can cast, but they can use each spell level's mirror spell multiple times if they have more than one spell slot at that level. Each "half" of a mirror spell is usually a spell of the same level, but if the halves have different levels the mirror spell has the same spell level as the highest level half. For example, a light/darkness mirror spell would be a second level spell, since it combines the 2nd level darkness spell and the 0 level light spell. If a mirror spell has material or focus components, it requires the components of both of its halves to cast.
Their are two main categories of mirror spell.
A reversible mirror spells are mirror spells that combine two existing spells whose effects cancel each other out. For example, a cure wounds spell cancels the effect of a cause wounds spell, cause blindness/deafness cancels remove blindness/deafness, freedom cancels imprisonment, stone to flesh cancels flesh to stone and so on. A kamerel spellcaster can combine any such pair of spells into a single mirror spell.
An invertible mirror spell is an existing spell with the Light, Darkness, Fire or Cold descriptor that can be cast as a spell with the opposite descriptor. For example, a fireball/coldball mirror spell can be cast as a Fire spell doing fire damage or a Cold spell doing cold damage. A sunbeam/darkbeam mirror spell can be a Light spell that does double damage to creatures sensitive to sunlight or a Darkness spell that does double damage to creatures sensitive to total darkness.