This is all awesome feedback, you guys definitely have me thinking.
I'm definitely interested in learning more about using the E collar, but like thecowboysgirl brought up, I'm very nervous about accidentally creating a negative association with the wrong thing. After he got attacked, he would panic if he saw even an on leash dog far away in the distance, he wouldn't allow strangers to pet him, and no matter where he went, he'd be shaking violently. It took so, so much to make him feel safer in the world and be able to interact with dogs and people without issue, and I'm very protective of keeping it that way!
I would definitely want assistance from an experienced trainer, I'm glad to hear it can be done. But that leads to another question... if I went that route, how would I go about training him with the meaning of the E collar in the first place, when his recall is solid everywhere else? He's regularly around horses, cows, goats, chickens, deer, etc. He knows to never chase any of them. The only critter he chases is a squirrel, and he'll still call off those every time. He's also recalled perfectly every other time we've seen a coyote, where it was slightly further away and neither approaching us or running for its life, so I'm not sure a decoy would make a difference, but I like the idea! I wish I could recreate the exact scenario more easily... close encounter, things get tense, animal flees. My curiosity is peaked by the idea of practicing him waiting for instruction while I actively engage with something the same way I did with the coyote, like heartandsoul mentioned. I wouldn't want to bring that energy to any place he's already nervous, but maybe in a very controlled environment? I work in law enforcement, I could definitely pick the brains of some of our more experienced k9 handlers or their trainer about that, since it's a huge part of their training. Would an E collar trainer simply have a session where they explain to me how to use it properly for him, or I would be paying someone to accompany me on hikes until we happen to come across a coyote to chase? Sorry for so many questions, this aspect of training is new to me and I'm very interested in whether or not it could work for him!
As far as what motivates him... I would LOVE any suggestions for an irresistible treat option. The only treat he'll eat is freeze dried beef heart, and even then he's bored about it. He loves his toys, but he's indifferent to tug and fetch. All he wants to do is play his version of "tag". I did use that form of play when teaching him recall and "leave it" initially, but the instant we go on a hike he couldn't care less about any of his toys. All he cares about is squirrels, playing with my other dog, and stressing me out by climbing trees like a leopard. I was toying with the idea of feeding him raw and ordered him some raw beef liver to see if he was at all interested before investing in switching him over... he gave it a couple nibbles and then just left it to rot. I've never heard of the food chase, I'll look that up now!
I can't tell you all enough how much I appreciate the help and the brainstorming! I was feeling so discouraged and this is giving me a lot more hope that there are options that don't take away our off leash freedom!