Concerning page-error.tcl and templating: With OpenACS 5.9, everything included via
<include> can be customized in the theme package (configurable via /admin/configure) without extra parameters. On OpenACS.org we have e.g.
openacs-bootstrap3-theme/resources/templates/packages/notifications/lib/notification-widget.adp
openacs-bootstrap3-theme/resources/templates/packages/notifications/lib/notification-widget.tcl
openacs-bootstrap3-theme/resources/templates/packages/acs-admin/www/apm/repository-index.adp
openacs-bootstrap3-theme/resources/templates/packages/acs-admin/www/apm/repository-channel-index.adp
openacs-bootstrap3-theme/resources/templates/packages/search/lib/navbar.tcl
openacs-bootstrap3-theme/resources/templates/packages/search/lib/navbar.adp
openacs-bootstrap3-theme/resources/templates/packages/forums/lib/search/search-form.tcl
openacs-bootstrap3-theme/resources/templates/packages/forums/lib/search/search-form.adp
openacs-bootstrap3-theme/resources/templates/packages/acs-subsite/lib/login.tcl
openacs-bootstrap3-theme/resources/templates/packages/acs-subsite/lib/login.adp
...
The ResourceDir is set via the theme package, and can be checked/temporarily altered in the parameters of the subsite.