(a)Yes, I had to go through several to find one that was comfortable, but THEY ALL retained my lower teeth perfectly. I actually have to wear it 24 hours/daily, so I'm not sure if night wear only would have been different? Unfortunately, I haven't worn anything on my upper for many years, and I definitely should have at least at night! My upper teeth are still perfectly straight, but stuff definitely shifted even though it's not visible. I found a bunch of my old upper retainers and appliances, and not one of them fits. Too tight.
(b)An appliance was what messed up my bite to begin with (it was the Twin Block, upper and lower) so I can't comment on the open bite issue. After this initial mess up, my bite hasn't gotten worse, though.
(c)I had to go through several doctors and several brands/types of appliances to find one that was comfortable. The one I have now is the ONLY one I've found to be satisfactory. Actually, it's completely fine where the others were torture. I just wear it on my lower teeth, to give me a bite after the Twin Block whoops, and I seems to be an acrylic type material. It's not weak like an Essix as I can eat in mine (dried mangoes, etc.). It wraps around both the front and back of my lower teeth, but only covers the tops of my molars. It does NOT cover the tops of my front, lower teeth.
I have an appointment to have a new one made in November. If you want me to ask the doctor any questions, let me know.
Take care
