In the events core.posting_modify_submit_post_before and core.posting_modify_submit_post_after, the parameter 'error' does not have any impact in further execution, and particularly it does not achieve what is specified in the docblock: "Any error strings; a non-empty array aborts form submission". Similarly, the parameter 'submit' is useless here as well, as both events are within an "if $submit", therefore this parameter will always be true, and changing it will not have further impact.
Please, remove both parameters from these events.