verifying the - against the + is not a double check at all.
You seem to be digging yourself a great big hole here. Please read this line back.
Even anecdotally it's dismissible.
- You send a transaction to Poloniex using a blockchain address they supply
- your local simplewallet app reports your balance reduced by the transaction amount
- But the balance on your exchange account doesn't move for a whole day
You are totally missing the point again. Your "balance on an exchange" has nothing to do with a cryptocurrency, but is something that the guys on an exchange decide to put on their website. They can change that balance at any moment, by just editing their web site database. The balance on an exchange is not "holding crypto", it is holding IOU on a website. So this has nothing to do with it.
Your local simplewallet application is a thing that takes the block chain, as I explained, and CALCULATES a balance from all the valid transactions on the chain. If that calculation is wrong because your application doesn't work correctly, then it is just doing a wrong calculation, but that *balance* is nowhere explicitly on a block chain. Only TRANSACTIONS are, from which software can calculate balances.
And it is in the nature of a transaction to contain BOTH an instruction of "augmenting a balance" and an instruction of "diminishing a balance". So from the moment that there is a valid transaction, AUTOMATICALLY there will be an augmented balance somewhere, and a diminished balance somewhere. Whether software, of websites reflecting IOU, or whatever DO THIS CALCULATION CORRECTLY, is an entirely different issue, that has nothing to do with the block chain itself. This software can work correctly, or wrongly, as well on the bitcoin block chain, as on the monero block chain.
You can just as well send bitcoins to poloniex, and NOT see your IOU account of bitcoins there increased, as you can send monero to poloniex, and not see your IOU account of monero increase. If poloniex's software is not running correctly (on purpose or with a bug) then that has nothing to do with the nature of the block chain.
If your bitcoin core wallet is not treating transactions on the bitcoin chain correctly, then your balances will not be correct, just as if simplewallet is making mistakes reading the monero block chain, it will display wrong balances. Because in both cases, these balances are calculated on the basis of the block chain transactions.
Your reference to your "scientific control" which I explained, was totally erroneous, because just testing twice the effect of a transaction, whether valid or not, is not bringing anything new.
Again: a transaction contains BOTH a "plus" for one balance, and a "minus" for another balance. OF COURSE if you include that transaction in the calculation of the first balance, you will see an augmentation, and if you include THAT SAME TRANSACTION in the calculation of the second balance, you will see a corresponding decrease. But that doesn't check, prove, or double check anything.
If the transaction is valid, which is the thing that must be checked, your balances will correspond AUTOMATICALLY (in monero just as well as in bitcoin) ; and if your transaction is invalid, the erroneously calculated balances on the basis of that invalid transaction will ALSO check. So the fact of seeing an augmentation of A and a decrease of B doesn't prove ANYTHING. It is a logical consequence of accepting as valid a transaction.
With a correct calculation of balances from a block chain, and valid transactions, your scenario "A is diminished, but B didn't increase" CAN NEVER HAPPEN. Because it is one and the same element (the valid transaction) that causes BOTH. If the transaction is valid, then the correct calculation of A will be diminished, and that same transaction will cause the correct calculation of B to be augmented. It is simply not possible that one and the same block chain, with the same set of transactions, would ever result in A diminishing, and B not augmenting, because it is one and the same instruction (the transaction). At least if the transaction is correctly structured, which is part of its cryptographic validation.
Your scenario is only possible if people are having DIFFERENT BLOCK CHAINS with different sets of transactions, but that is also true for bitcoin.
The trivial other cases can be when your wallet software makes a wrong calculation of balances (that doesn't affect anybody, because that doesn't modify the block chain), or when other software writes wrong calculations in a web site database. But that can just as well happen with bitcoin, as with monero, and has nothing to do with the currency principles itself.