First of all, great plugin! Bravo!:)
Small note "however", if you have class or id into <h2>, h3>, etc (i.e <h2 class="title-page">... them these titles will not be taken into account. (In that case, only the TOC title will be displayed).
I spend "some" time to find it, so I guess other should, and/or you could add a note ;) (I'll personally remove the class from title as I don't use them anymore ;).
BTW, I can't remember where, but you got problem with WP Super Cache being deactivated, but pages are still cached... Ask the guy to NOT deactivate the plugin, but into "WP Super Cache" settings, "Easy", and ask him to check "deactivate cache" ;)
BR - Bonne semaine - Franck