Make wordpress comments work with include templatepath

Short answer: you can’t.

Longer answer:

You can include comments.php as a template-part file inside another template, via:

get_template_part( 'comments.php' )

…but that won’t actually make comments work, because the comments_template() template tag does far more than merely include the comments.php template-part file. In order to make comments actually work when using get_template_part() as opposed to comments_template(), you’d need to duplicate all of the functions performed by comments_template(). And if you need to duplicate all of that code in order to make comments work, you might as well just use the function itself.