Hmmm... I think I'll make my character a Level 4 or 5 Illusion Mage who has an obsession with ringtails and illusioning people into them, if I understood that part correctly. The only reason he hasn't been killed yet is that the wide-ranging AOE of his spells hides him well.

That is a valid strategy for a level 4 or a level 5 to exhibit. To basically use their own abilities to hide themselves from those that would be looking at them. There are factions that will make note of every action a level 4 or 5 mage takes, but that is only if they figure out that that mage is at the level 4 or 5 state. A mage that can go to level 5 but never exhibits anything above a 2 or 3 won't draw their attention. And not all of them are hunted down anyway. It mainly boils down to how big of a threat they are. A level 4 mage that decides to take over a country in his own realm and never shows any inclination to using the portals will get a response from the church and academy closer to a "okay then, we'll just avoid that area." Similarly, a level 5 healing specialist that is just going around to all the realms healing the sick wherever they go also won't really generate that much of a response from any of the groups. It's just 4's and 5's are so difficult for any single person to deal with, that if they are a threat to that group's plans, then steps must be made to take them out as quickly as possible.

Wow, that moment when you stumble upon not just a legitimate strategy, but a good one, and get the main author to expand upon it... That's satisfying.

And considering you didn't correct me on the use of Illusion magic to make things transform, I take it I understood it correctly?

Yes, you can use illusion magics to make people look like they transformed. You could quite easily use an illusion spell to make anything look like anything. It just doesn't change what that thing actually is. You could make a flower look like a steel beam but place anything on it and it'll still go squish (though it could be made to look like it was placed on top just fine until the spell is ended at which point it's still a squished flower). So you could transform someone into another species with an illusion and depending on how thorough it is will make them look, sound, feel and smell like the new species. But their actual species would not have actually changed and when the spell ends so too will the effect (since as stated, magic effects only persist as long as the mage is maintaining the effect. Effects caused by the magic (like burns from getting hit with a fireball), however, would continue after the spell is ended).

As for how resistance and scaling would work with such a spell. For now at least I'm going to keep stuff like that somewhat vague unless it becomes necessary. Partly so that it isn't suddenly making everything get bogged down by me having to go spell by spell of "what does it mean for someone to resist healing magic a bit," etc. and partly because I feel it restricts creative control from the authors. Unless it becomes a problem where people abuse with things like "well they should have ~50% resistance to it, so that just means it'll fail half the time and I have to cast it twice to have the full effect" that defeat the point of the target being able to resist I think it's a good prompt to leave it to the writer. Like for your illusionist: perhaps resistance restricts how much they can change or how much effort it takes to produce and keep the same level of effect or perhaps there's a time limit on the spell that is reduced by someone resisting or the distance they have to stay close to you and if they exceed that distance it breaks or even how many senses it affects. And I feel those kinds of choices add just so much more depth to the characters and story than me just going "resistance means it does x"