I usually hear positive things about Nintendo customer support so that's disappointing to hear. I am guessing you sent it off for repair? I would suggest working on paper to avoid getting low level customer service agents and keeping a log of your interactions.
If Nintendo does not have the parts there's very little they can do besides refund you. They seem to be at Nintendo Japan's behest when it comes to this stuff.
May be worth researching your rights when it comes to repairs, and if there is any reasonable time frame Nintendo has to act in. You could cite these rights, or go through citizens advice yeah. But I would leave CA until a last resort tbh. They're usually pretty busy helping people who have lost a lot more.
Apologies if there is more to this, but surely the best thing to do is return to retailer for refund or swap under the Consumer Rights Act (previously called Sale of Goods Act)?
Retailer won't transfer saves because Nintendo didn't bother to implement that feature yet. I don't know if you put 100 hours into Zelda or something but I'd probably just cut my losses and take the refund after another few weeks.