My response is probably not going to answer your questions, nut here is my experience with them.
I use HME's 4 days a week almost every week.
First, an HME will never feel as good as a real humidifier...that is just not going to happen. Also, I do not think that the manufacturers would recommend using any water and I never have to.
In more humid environments I have used an HME for about 14 consecutive days and still felt comfortable. Also, when not actually in use, remove the HME and put it in a zip lock bag.
While it may not be applicable to this trip, if you're in a hotel as opposed to a hostel or camping, run the shower on hot until you see the mirrors in the main room start to fog. I found that helps in dry climates and I can even sleep without a humidifier when I do that although not as comfortably as with an HME (d it when I forget to pack one)
There is no question as to whether HME's work, they've been used for years, it is just a matter getting used to them.