Was your illegal offset error in the panel or on the site itself? I have had issues with the most recent kirby version and the builder plugin, illegal offset included. Try down grading Kirby to 2.5.3 or 2.5.4. Hopefully the builder plugin will be updated soon.
I couldn’t access $page in the same way you do in a front side template in the panel either. Try:
$page->site()->index()
Thread about it here: $site variable not working in Builder plugin snippet