Hey Vadim,
I really can't answer those questions man.
I don't know what type of "warranty policy" you've got there and I don't know what exactly went wrong, when you were trying to fix your Casio.
. Simply take your keyboard to the shop and let them fix it,
They might even tell you if that warranty paper that you've got is valid (or not).
In a worse case scenario it would cost you like $50.00 to fix that key. But if the key is OK, and the key contact is warn out or damaged - it's like $25.00 to fix. (depending on how much of an asshole the repair guy is
)
Oh, I had a similar situation with my JD800 (screwed up aftertouch strip) and I had no warranty papers. Roland tek simply looked at the serial number and it turned out that my warranty was still valid. I'm not sure if Roland still use their serial numbers to find out when the unit was made.
But really nothing can go wrong if you take your Roland to the shop. It's the safest way to do it. (Especially if you have a history of breaking Casios
)
Best regards!
-ED-