Thanks for supporting this rather odd situation, @2020matters!
I have made some progress:
I was able to complete a transaction as intended (but only with PayPal, which is OK) by using a VPN set to France.
Sébastien explained in no uncertain terms that Qobuz forbids the use of VPNs and that, if one is detected again, my account would be closed and they would not consider any kind of appeal!
Message understood.
I still feel as though this is a network issue. Something in their (third party) merchant setup is defeating transactions as such.
But of almost equal concern is the loop which I (at least - as in my post above) encounter when trying to get a purchase into the cart and pay for it (albeit with PayPal).
Yesterday, however, I tried to do the whole thing from scratch. Restart. FireFox (and of course no VPN).
It worked flawlessly. I was able to complete the purchase and initiate the download of eight tracks on a high-res CD and have the FLAC files in the right directory to import into Roon in about 15 minutes.
This makes me think that - even though I deleted cookies and disabled all other trackers, blockers etc - perhaps once you've started a purchase trying to use Visa etc, misconfigurations or bugs in the Qobuz system shut you out and result in the loop I described above.
I'm waiting for Sébastien's response to what for me seems like good news.
But if I'm honest, like you, I still don't really know where I satin or what to expect.