Sounds like very odd advice from support.
Can't see how most of that would make any difference.
When's the last time that a transaction on 48 using that card worked for you?
Is it only on the gift store that transactions are failing?
Do other transactions work ok?
E.g. topping up your own cash credit balance or buying a new membership?
Can you gift the other person the cash credit to buy the membership rather than trying to gift the membership directly?
Can you try a different browser and/or device?
Hope this helps.