"Users can't see feature"
https://doc.arcgis.com/en/arcgis-online/manage-data/manage-editing-hfl.htm#ESRI_SECTION1_C30D73392D9...
There's an editing setting so that users can't see any features, even those they add. It's recommended for editable layers shared publicly, as it will allow adds but not updates or deletes. Then create additional (no editing) views for access.
"Invalid Name" error (probably because "GlobalID" is a reserved keyword and it thinks i'm making a new GlobalID?)
parentglobalid is not a reserved word.
You may need to target the related table (form_id setting) and drop the parent questions from the survey. So that you target the related table only in your survey form.
Theres' a (very old!) video showing the concept below. The core workflow should be the same but you'll need to ignore the 7 year old UI and talk of upcoming (now existing) functionality:
https://youtu.be/vECznnoJHGk?t=2528
Also the survey should be accessible alone without needing to navigate through field maps.
Survey123 is designed to ADD data. If you want to add just related records, you need some way to connect it back to the parent. You can use inbox, custom URL as above (from ANY link or field maps, as long as it has parentglobalid), or manually update the survey form so that there's a pulldata expression to calculate parentglobalid.