Hi Dan
You can edit existing features using the Survey123 inbox. In your workflow, you would need to run an inbox query to pull down the collector submitted features/records, and then select the relevant feature/related table in the inbox ready for editing. You would then be able to add new records to the related tables or update existing values.
We do have an enhancement open to allow you to use a custom url to download/open a specified feature in the inbox - and this would make the process more streamlined, as you could use a link to get to the record.
I note that you are creating both parent and child features when submitting data via your surveys. It is also possible to create a survey that targets the related table directly, meaning that you can add inspections to the related table without submitting a duplicate of the asset (to the feature service). You could, in fact, use the custom url from Collector to launch a new survey targetting the related table, prepopulated with the GUID used to link it back to the parent feature. I can provide more information on this workflow if required.
Thanks
John