Parent record for viewport & sub tabs, child record for page

Parent record one-to-many Child records.

I have a list page for Parent records.
I click a Parent record.
Edit Parent has its own Edit Parent viewport.
The viewport contains multiple sub tabs, each with a single page under them.
Click through those pages and the menu remains available and the viewport can show nuggets of the Parent record’s basic info.
One of the sub tabs lists Child records relating to this Parent record.
This particular sub tab contains one page for the list of Child records and another page for adding an a new Child record to this Parent.
Since the Add Child page has a base object of the Parent record, this can still remain in the same viewport and so the viewport nuggets and Parent menu sub tabs remain and work - great.
The Child list also has Edit buttons.
Since the Edit page needs to have a Child record as the base object, it needs to be a separate page to the Add form. Annoying but I can live with that.
What’s annoying is that because it is a base object that’s not the Parent then it needs its own viewport so the same Parent info nuggets can be displayed by going up the relationship from Child to Parent for each info item because the screen doesn’t know the Parent record.
What’s just as annoying is that because the screen is working with the Child record, I don’t seem to be able to show the Parent record’s menu sub tabs.

I’ve tried a variety of links ([Auto], Context Record, New Relation Existing Relation) from the list page to the Edit page and different objects on different pages, sub tabs and viewports but unless I’m doing something wrong I can’t manage to get this to work.

I’d like the screen to always show the viewport with Parent info nuggets and all the Parent record sub tabs including when on the Child Edit page.

Has anyone managed to do this or is it not possible to have a page that effectively references two [related] records so a form on the page can work with the Child record, and the sub tabs and viewport can reference the Parent record?
This would help to ensure a consistent UI for the user when adding or editing Child records to a Parent.

The number of fields and complexity of the form on the Add & Edit pages means that they need to be a page rather than a popup or a form alongside the List (widget group)