Wow, this is a really tough dog!
I would have been surprised if Melatonin worked although I do like it for other situations.
Your options are going to be limited but I keep thinking about your muzzle comment.
A properly applied muzzle shouldn't come off in most cases.
Here's one thing to consider:
Try another antihistamine.
Get the muzzle on him and make sure it's really tight (you're not going to choke or smother him).
Wrap him in a towel so he can't get his front feet around his face to take off the muzzle.
Hold him really tight no matter how much he squirms.
Then have a second person locate his legs
/feet and hold them tight feet while a third one trims his nails...or trys to anyway.
I've dealt with a lot of small dogs (and some fairly large ones, too) who absolutely HATE to have their feet/nails touched. In those cases, if I can't manage to trim their nails with the method I just described, then I dispense drugs for the owners to use.
Unfortuantely, there aren't going to be any other drugs available to you without a prescription which your vet would have to provide for you.
I'm really sorry that I can't provide you with a more positive answer. Deb