First understand Ive not kept parsons before, nor have I used these types of "swamp coolers". I have seen them used to cool and add humidity to rooms that need it though.
It would help cool and raise humidity, yes.
You can make it in a wall, and bring in the fresh outside air, or build a smaller portable one, and place it where you like.
Heres one someone built:
You are asking that if your humidity is ideal, but temps are high, if there is a way to just lower temps without messing up your humidity?
Then, yes.
Simply turning off your basking light should drop temps, and not touch your humidity. Your humidity may go up a bit even.
Misting the enclosure will drop temps a good deal, through the ssame principle as the swamp cooler, evaporative cooling.
A fan in the room will help cool things to some extent, but you may loose a bit of humidity.