Sounds like someone is trolling...
If you're legit here's a solution, give it to me.
I agree must be a troll. Especially considering the post count.
Anyway if you are legit then I suggest you go and read your warranty properly.
Most companies wont cover you if you watercool your card so that would suggest that they will check your card which would seem like standard practice. If they see that the card has been tampered with or see that the card failed in an unusual way then they will not cover your card. They will like most companies see a bit of stock returned and will have a list of your typical warranty items and a list of what they will not cover. Eg a cars warranty will cover things like engine and gearboxes etc but will not cover clutches or brakes as they are deemed "perishable items" ie they wear out over time and use so they are not covered.
They idea of trying to put the card into the microwave is just idiotic. If you have ever accidentally put anything metallic in ie alu foil in then you will see burn marks all over it from the heat. This will happen to your card too and when you send it back they manufacturer will turn around and say go away you obviously tampered with the card.
Besides all that there is no guarantee you will get an improved card. What you will most likely get is a "B" grade or "old stock" or even a refurbished 590.
As for the money you spent you gotta be an idiot to try and justify you doing this because you wont get what you paid for it. That is life. That is the way everything works. Once you use it, open the box etc the value goes down. Why do you think they have a saying for cars "Once you drive it off the forecourt you loose 2k off its value".
It is called depreciation. It happens with everything. It is due to time and also the fact you have a new product replacing it.
My advice it keep the card and wait until you can afford a proper upgrade and you will be happier. You also do not run the risk of getting caught for fraud.