OK, after weeks of struggling and the Starbucks chat and phone agents not being much of a help (they could only UNLINK the two accounts, but not LINK them for me!), I finally figured it out.

Given that there are still two days left for the promotion, somebody might still benefit from my findings:

I succeed using the internal VPN function (the icon in the address bar) of Opera desktop browser for Windows.

But it would not work if I wanted to assign the Delta number to existing Starbucks profile. It would only work if I first went to deltastarbucks.com,
validated the DL number and then CREATE NEW Starbucks account in the second step.

And it would only work with a .com address, do not use any national domain.

i could not log into the Starbucks account afterwards (well, it did work ONCE!) but the mileage bonus was assigned instantly.

Hope this helps someone.
Happy new year.