You've got a point, the root of the tree could be made available to the main charting sites.. or even weekly written into the blockchain.
As for the negative values, I wasn't thinking of robbing anyone, but just pretending you are solvent when really you're not. I'm not sure I see what you mean by "swap out which customers you're robbing", could you expand?
E.g. say you have two customers with a balance of 100. You report the total is 100— so 100 BTC has gone missing.
When customer A logs in you give them one root and show them their balance (and B has a balance of 0). When customer B logs in— oops balances just update— you show them a new root, and in that one B has a balance of 100.
So you need to pin the commitments strongly enough so that the prover can't swap them out at will.
As for the negative values, I wasn't thinking of robbing anyone, but just pretending you are solvent when really you're not. I'm not sure I see what you mean by "swap out which customers you're robbing", could you expand?