If ever I lose a certain amount of Bitcoin (large or small), how can I track it down or claim it back? Who should I contact?
I don't think you understand what decentralized means...
You contact the CEO or Bitcoin Mark Karpeles.
Joking aside BTC requires the user to have a little computer knowledge about how to secure your coins. Until a relatively foolproof and cheap device is made to secure coins the due diligence is on the part of the coin owner.