In the 2nd test, I can finally see pageTitle1 in page creation and after creation (Open page properties) BUT all the other default fields (example: navTitle) have disappeared. Instead of extending moretitles, it seems I've overridden/replaced it.
(contents of /libs/wcm/foundation/components/basicpage/v1/basicpage/tabs/basic/items/column/items/moretitles)
As you are trying to add a new field in the fieldset you will need to overlay the node under 'apps' folder. Overlay should be created using the below path: /apps/wcm/foundation/components/basicpage/v1/basicpage/tabs/basic/items/column/items/moretitles/items/custompropertynamehere