I haven't had a book disappear from my Glo, but I only sideload via Calibre, and don't use Kepub on my device at all; nor ADE loaded either.
But - I read my Glo the other evening when it was 41C in my lounge room (un-airconditioned house) in Sydney. It was the only light on in the house - that is, every light in the house was off, other family were sitting outdoors, and I was inside using the Glo to read.
And, wonderfully, the Glo didn't seem to get hot!!!
I took it out of it's jacket, to allow for any air circulation (and there basically wasn't any for most of the time), and I leaned it partly on a cushion in a tilting fashion, which sat in front and to the side of me so that I didn't strain to read it or touch it for page turns.
Good luck, and please report and let us know how it went.