As for the first problem, something you’re doing is causing a PHP error. You can try enabling debugging, you need to see what the PHP warning is before the error you posted to figure out where there problem is.
As for the second part of the question, my suggestion would be to use acf_form with the form option set to false. This would output the fields without the <form> tags. Then build AJAX to do the needed updating. This will be pretty complicate to be honest and I’m not sure of all of the things you need to deal with though and it may take even more modification since you’ll need to somehow associate each field with the correct post. In the end though, it would create a better user experience.
