Does the LED only turn on when the device is misting or is there a way to just have the LED be on (either color cycling or static color) without the device misting? Thinking of this as dual functioning as a night light…
You can use it just as a night light as well, or just as a mist, or together
What are the dimensions of it?
Can you have it be on a static color or does it only cycle through the colors?
7 x 7 x 9 inches
9 x 9 x 12
Yes
I have one of these and it works well. The reservoir doesn’t last long, even on lowest setting, so you’ll need to fill every 1-2 days. That’s based on overnight use only. If you’re running it longer, you will likely need to refill more than once a day. The lights are nice and it works well as a night light. It looks nice. Parts can be tough to clean with some sharp, hard to reach corners on the internals, but overall, I’m happy with it. I think I paid somewhere around $30 for it, so be wary of that $80 start price. <$20 is a good value.
Does this humidifier have any filters that need to be replaced?
How big is the water reservoir?
After 500 hours its recommended to replace filter
2.8 Liters, Will work for 10 hours straight without refilling
Can this be used to diffuse essential oils?
Yes
I have very hard water. Will the mist leave white powdery spots on nearby furniture?
I have hard water and the sediment does not get on my furniture but rather builds up in the lower base.
Is this humidifier UL Listed?
yes, it’s UL listed. We’ve updated the listing to reflect this. 
How much are the filter replacements? Can you get them thru Amazon?