When my wife took Bella in to see the vet, the vet did a pretty extensive exam. She could find no evidence of mites, but there was a possible faint line at the base (possibly hinting at injury?). Because Bella's ear was flopped down at the base, and she could find no conclusive reason for it at the time of the examination, she took the time to go to whatever computer resource they use at their office to see what she could find out about drooping ears. Other than what we already could guess at (ear infection, mites, heat, injury, genetic weakness), she didn't find anything else. She put Bella on antibiotics and a probiotic to cover a possible middle ear infection, and suggested we try to find a way to splint her ear to the other to support it in case of an injury. One suggestion was trying a sensitive-skin tape at the tip of the ears, but this did not work, just did not stick. My wife also tried wrapping gauze around the ears, but as soon as Bella flicked her ear, it was off. She is almost at the end of her antibiotic treatment, and she holds her ear up on occasion. It doesn't seem to bother her, or slow her down at all.
Thanks for all the replies.