Manufacturers cannot deny a warranty even when they claim they can.
Panasonic doesn't even run the ridiculous game of claiming they can circumvent federal law by whim and deny you a warranty.
Get a receipt on a sealed-box product and you have a warranty. Also, if you're a Costco member, your $99 insurance policy of a SquareTrade warranty is not a terrible idea. (I believe even non-members can buy from there for not much more on Costco.com but you'd have to verify that yourself.)
There is no difference in HDMI cables. If you can see the picture without visible dropouts or sparklies, the cable is working at 100%. No other cable will display a better version of that picture. You're simply wrong if you think there is a better digital cable than one that is already working.