First off let me say that I am glad Max is still amongst us and obviously fitter than you might wish to believe because he did resist your ideas. I already love this little smart fellow.
Reading all of what you wrote I learn that Max comes to you, eats and allows you to pet him. He allows you to rub his belly and all this which is just lovely.
I learn you used a sedative to calm Max down (why?) - you did pick him up and put him on your lap to pet him .... and I am (from what I read without having been there on the spot) asking myself why not pick him up and put him into the carrier right away? And yes, he might have fought you, he might have scratched or bitten you even.
I once had to take a cat (his human mom had died) out of his territory outdoors and take him to a vet for check up and then to a very nice shelter. I did plan this very well beforehand and it was not funny at all, I wore leather gloves and I really did grab him very quickly and forced him into the carrier which was opened on top not on the side (which I imagine more difficult). He was surely furious all the way to the vet and raged in this carrier like crazy. He made it another year at the shelter and grew to be 20.
In contrast to the cat I surely saved, Max is a feral cat and he will never follow what you have in mind for him, this should be very clear to you. Have you thought about what to do next if the vet tells you he is still quite fit and will make it for some time? Do you know where to put him? Can you take care of him? Those are important questions because if you cant take care of him after “saving” him, you might just consider to let him be.
I dont know if you have asked your vet about Acepromozine, what it is, how it works and what side effects it has. Maybe you should google it on the net to get more informations about this sedative which does NOT make the pet unconcious but is thought to calm the pet down, meaning here the pet cant move but is very aware of what is happening.
My prayers go out for you. If you do not find him again tomorrow – please do not worry so much. As I told you ever so often, keep in mind that Max is a feral cat. He might be old and ill but he knows his way in his territory and will surely escape from all the odds out there just like he did escape from your approach to catch him even with quite some sedatives in his system. And like I told you many times before – yes, he might just die out there like all feral animals do.
It is so great that he trusts you enough to spend time with you.
I sincerely hope you dont mind my thoughts on feral animals and I know how difficult and painful all this is for you.
I hug you