Partner Center : Product submission : In draft : stuck on a single incomplete section

We have been trying to submit product release details for a game.

We have managed to enter all the relevant details (10 different languages, a host of images and videos, texts etc.), upload the package and have all sections marked 'Complete.'

However, whatever we do and try out various options with 'Properties,' those relentlessly persist 'Incomplete.'

Some posts suggest logging out and then in again, clearing browser cache, cookies, files etc. Nevertheless, this problem persists also after logging in via a completely new/different browser afresh.

The biggest frustration arises from the fact that only a few bits are marked as mandatory in the 'Properties' form (as is also indicated in the relevant documentation) and those have been filled in. There are also no marks pointing to inadequate entries. For some reason one has a feeling the records are not getting properly refreshed upon saving. E.g. 'split screen' continues to appear checked on opening, although it's unchecked before saving. It's similar with privacy policy (a link vs. a text entry), those bits have no way of completely clearing.

1. Does anybody have a solid idea as to what might be the issue?
2. Has anybody successfully overcome such an issue and how?
3. Eventually, any idea how to directly contact Microsoft Store Support and get their help? (It seems an impossible endeavour, all links are either circular or a dead end.)
4. Can these problems be avoided using Partner Center API and automated submissions?!? (It seems complicated to the inexperienced.)

P.S. 'Xbox Creators Program' is marked 'not started' as we do not target Xbox.