Apologies if the following point has already been discussed ad infinitum elsewhere, but I've noticed that whenever I have sent BTC, the transaction has always been published instantly, even though it took a few minutes to actually confirm. If the Bank of America or Visa were to use the BTClockchain for their own internal purposes, they would certainly have procedures in place to prevent their employees from trying to double-spend the BTC sent to addresses to establish transactions. Since such published transactions are sure to be confirmed, shouldn't publication alone be sufficient both for legally accountable record-keeping and to initiate or complete smart contracts? If so, doesn't this mean that, for such uses by such institutions, there is no transaction-time limitation?
No. Even if a transactions is broadcast, there is no guarantee that it will confirm and thus become irreversible. Sometimes transactions have some issues like low fees or dust outputs which prevent that transaction from being confirmed. With the upcoming Opt-In RBF feature, it may not be safe to take unconfirmed transactions if it has opted in to be able to be replaced by another transactions with a higher fee.