UPDATE: I'm going to update this one up at the top, because the solution I've found was with a differing filter hook. I'm going to look again at reply_link-specific hooks now that I better understand how to use them, but the solution to the main problem turned out to be easy once I found the right one: 'thread_comments_depth_max', which sets the default highest maximum depth to be set in Admin/Settings/Discussion.
Rather than explain it in detail, I'll link you to my github on the freshly minted plug-in.
https://github.com/CKMacLeod/WordPress-Nested-Comments-Unbound
For now, the rest of this question is overall obsolete, though kind of a record of alternative ways of achieving the same thing with greater difficulty and less general applicability.
Since 4.1, WordPress has made the filter "comment_reply_link_args" available, apparently (?) with the intention of letting us intercept and modify particular options normally set in the admin panel as they apply to comment reply links, while leaving everything else intact.
Though there are many aspects of the reply link that one might want to modify via alteration of the "args," the one that prompted this investigation on my part was max_depth, so I'll use it as my prime example.
When a nested comment thread reaches its maximum depth, the default functionality is for the comment reply link to disappear, but, on a comment thread with an active discussion at maximum_depth, continued conversation can be much easier to continue if the reply link continues to appear at the bottom of every comment - or alternatively, though a more complex implementation, at the bottom only of the thread - as if the comment thread at max-depth becomes a simple columnar/chronological thread.
The first alternative is easy to achieve via a hack of comments_template.php (linked below), deleting two lines:
if ( 0 == $args['depth'] || $args['max_depth'] <= $args['depth'] ) {
return;
}
However, since one of course does not want to hack WordPress core, the more common, preferred way to accomplish this end would be via theme or child-theme files and application of a callback comments function. Given how many different customized comment forms there are in different themes, this option might be preferable. However, if all you wanted to do was offer the "infinite replies" option for any theme or design using common WP functions, then the preferred method would be, if I am not mistaken, to filter the above $args where they hook up with comment reply link function(s).
In order to achieve the desired effect, or the simple version of it, a theme modification that works with an elaborate custom callback function is:
comment_reply_link( array_merge( $args,array(
‘depth’ => $depth,
‘max_depth’ => $args[‘max_depth’] + 1,
) ) );
I am copying everything except for the +1 modification as found to work applied to an actual theme's callback function. I am not completely sure about how all of the variables are declared, since, in my attempts to render the above as a filtering function used in a different theme, I continually ran into missing argument and argument not array errors.
Leaving off more complicated attempts to correct those errors, in a simple world the following would work:
function infinite_replies() {
$comment_reply_link_args = array(
‘max_depth’ => $args[‘max_depth’] + 1
);
return $comment_reply_link_args;
}
add_filter('comment_reply_link_args','infinite_replies', $comment, $post);
It does succeed in obliterating the reply link altogether - but nothing more. But at least it doesn't produce error messages... so I guess that's something.
I hope that the solution to this problem will point to ways to achieve other, more complex, potentially theme-independent comment thread modifications.
For reference, the basic documentation on comment_reply_link_args is here - http://wpseek.com/hook/comment_reply_link_args/ - and points directly to the relevant portion of comment-template.php.
