I've figured out the problem, by the way. There were two issues. One was the overall_header.tpl file had some unclosed tags, like <table> and some <tr> tags. I had to get rid of those or close them. The other issue was I unknowingly copied and pasted the entire mod into the .php files TWICE - I thought I had edited into fresh copies, but had unwittingly modified the files with duplicate entries of the same code. Oops.
Everything is working fine now! Thanks for the feedback and help.