Hmmmm, that's not my understanding of the way section 75 works, although it is what I might expect a credit card company to try and say. The law is very clear on this - they are jointly responsible and you have a legal right to redress. Many also try to bring risk management into the equation by saying your purchase was risky, but again, the law is clear. It's the price they pay for being able to charge interest and fees.
I can certainly demonstrate that I have taken all reasonable steps to get it repaired, including paying for shipping for repair as well as conversations with the supplier, but there is actually no requirement of a 'first point of call'. There is no requirement within the law that talks about a 'fair chance of repair' either, although it does help with a credit card company's willingness to help. It doesn't matter how much the credit card company 'converses', they have no grounds to refuse.