I found the source of the problem why the auto insert wasn't working (and why manual editing of usercp_shell wasn't working). It was because i had
Icons for UserCP modification installed, and that modification loads a new usercp_shell. I uninstalled that modification and the auto insert works now. You should add that in your main post.
It would be great if this modification could support administrator set canned responses that specific usergroups could use (but not edit). For example, i have a reviews forum and reviewers need to manually copy the section headings to make the review, it would be great if they could select from a premade template with all the sections i define set for them.