The default maintenance page content can be updated from the Store ⟶ Configuration ⟶ Magediary ⟶ Store Maintenance Lite ⟶ Content ⟶ Maintenance page content Reference ...
To check the perview, click on the Preview button under the Store ⟶ Configuration ⟶ Magediary ⟶ Store Maintenance Lite ⟶ Content. Reference guide: https://magediary.com/docs/store-maintenance-lite.html#preview-maintenance-page
Yes, all selected items will be listed out on the frontend product view page. The customer can choose as many items as they want by selecting multiple checkboxes.
Tabs that matched Conditions will be shown on the product page. It also depends on the other fields like Status, Website, Customer Group, From-To, Discard subsequent tab.