Everytime I think I understand Bitcoin, things get fuzzy.

First, Bitcoins are not unique items, you cannot identify one Bitcoin from another, in fact that statement almost makes no sense.  Bitcoins are values that are assigned to addresses.  The addresses are unique and each address has a currently Bitcoin value and transactions transfer values between addresses.

Secondly,  it seems the whole system works because all the participants agree to play by the same rules.  What prevents a rule change if everyone agrees to it.  Changing how many bitcoins are given to miners, how much the difficulty changes, etc.  they are arbitrary.  What if a majority agree to a rule change?

Lastly, I read about Bitcoin theft.  If I have a wallet of addresses that is backed up.  A virus penetrates the wallet and transfers all my Bitcoins to some other address.  Since I have the original addresses, I can see where they went.  If I clearly demonstrate the above to the Bitcoin community, what's preventing the Bitcoin community from disallowing any transactions from any address that received my stolen coins?  Yes it would be difficult to convince everyone but is it theoretically possible?