B2B Collaboration Excel Entra invitations blocked due to suspicious activity

I started inviting external client collaborators to work on excel 365 spreadsheets yesterday - I have 20 spreadsheets in total that need to invite 20 separate collaborators (each person has 2 emails) and I sent out 6 of the invitations which worked, and then on the 7th one onwards I was prevented from inviting any new collaborators, the error message below came from the 'sharing' function in excel 365 itself.

I read the FAQs and went to check that my own account allowed admins to invite external guest collaborators (I was inviting from my global admin account - so that was fine). I then opened a support ticket from M365 admin centre (case number xxxxxxx) and the support person asked me to try add a new user using "invite user", which I did and a similar message blocked me below.

The key message seems to be "Invitations are blocked for this directory due to suspicious activity". We then went to the 'overview' section of 'Entra ID' in the 365 admin centre where there was supposed to be an 'unblock' option in the bottom half of the page (see screenshot below) but it was not there -

We went to the Azure portal and there were notifications with the same key message and suggesting I contact Microsoft support. I was reviewing all of this at the same with Microsoft Support who was on the phone with me and my screen was being shared with the gentleman (which to his credit, he was doing his best but we seemed to be hitting a lot of dead-ends).

We then went to Microsoft Defender to see if there were any reported incidents and there was nothing there. At this point my support person said it was an Entra issue so we went to the Entra Admin Centre and tried to raise a support ticket. He guided me through the parameters to input set out below.

After clicking-through we eventually came to a page that said I need to upgrade my plan in order to create a support request to the Azure developer support plan - if I did not there was no way to proceed, and my support person said future 'free' m365 admin centre support requests would just bring me to this point again because Azure is separate. So I agreed to pay and upgraded my subscription, and I was still not able to create a ticket - I was brought to a new final page referring to the Q&A community - see below. The "next" button is greyed out, and so is the final "4. review + create" support request is also greyed out and unreachable.

This was disappointing because immediately prior I was promised the ability to create a request if I agreed to pay, and then that option disappeared after payment so I felt misled to purchase something that was not delivered.

It seems that the blocking was triggered automatically and needs to be unblocked by someone at microsoft (not something I can unblock myself through my own account), and the only way to contact someone is not available. Meanwhile the document that needs to be shared and collaborated on does have time sensitivity, so I'd really appreciate help with this one. Thank you.