Sounds like a fun project, best of luck! Would love to see what you come up with. One word of warning with the misting: using them in an "upside down" configuration as I'm doing can be a bit finicky. I've encountered some that slowly drip due to the pressure of the water above them. Look for misting discs with small hole sizes (<=4uM) and expect that some percent of them won't work well. However, the ones that do drip at the start typically stop after a day or two of use and are fine from there on out.
I have not really considered decontamination and I don't think it's much of an issue for terrariums because the plants and soil in the enclosure already contain a lot of bacteria, etc. I could see it being much more important for your use case with mushrooms where a small number of mold spores can ruin an entire mushroom grow.
I'm prototyping my own similar kind of thing right now - an enclosure for growing culinary mushrooms - so seeing how you've used and attached the humidifier discs is really useful inspiration.
On humidification - have you considered decontamination for the water? I think bacteria and scum build up is an issue, and I've seen heavy duty equipment incorporate e.g. a submerged UVC light for trying to keep it clean. I wonder whether it's possible to do similar on a small scale by using a UVC LED (obviously need to be careful with safety though)