I cannot add graphs to a template screen. The popup says "No graphs defined."

The immediate cause of this behavior is in popup.php line 1358 ff., where the host selection from whatever a pageFilter is, overrides the one from the request parameters. Most likely, the override hostid from "pageFilter" is the previously selected "real" host.

This specific symptom can be fixed by skipping both the branches of the if statement on the next line if isset($only_hostid), but I cannot say whether this will break elsewhere.