Gezus that is some dodgy as hell transaction patterns, fnb not stopping it or alerting you to it is extremely weird.
Also, they should still be disputable. Whether they're debit or credit cards, they should still have the same authorization flow ie first a charge is placed on the card, then it gets settled a few days later. If you alerted them to it on the same day, they should have been able to do something about it.
Also, being a debit card doesn't mean that you can't do a chargeback. Visa's site says
"
Could you get your money back?
If you’ve paid with a Visa credit, debit or pre-paid card and your purchase has gone wrong, you may be entitled to ask for a refund. Your first step should always be to contact the seller, but if you're unable to resolve the situation to your satisfaction, your card provider may still be able to help you."
And regarding it being a wallet transaction
"
If you paid using a digital wallet
If your Visa card purchase was made using a digital wallet where the payment was made with a linked card, chargeback and Section 75 claims work in the same way as if you paid directly with your card.
If you paid with a linked Visa credit card, see the ‘I paid with a credit card’ section. If you paid with a linked debit or pre-paid card, please see the ‘I paid with a debit or pre-paid card’ section."
So it shouldn't be treated any differently to if it was a purchase with your regular card.
Learn all the need-to-knows around payment disputes and chargebacks, including when you're entitled to make a claim and how to go about doing so.
www.visa.co.uk
Also, from Visa's site
"
How our Zero Liability Policy works
Pay worry-free
You can pay securely with your Visa debit or credit card to make purchases both online and in-store locally or when travelling abroad. We have payment technology in-place to help detect and prevent scams and fraud, however, if it does happen, you have protection"
FNB should really be doing more for you