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1
According this this WikiPage, a MultiSigTransaction that doesn't include a fee may never get included in a block.
Since I've read that replacing Transactions (by version number or any other feature) isn't enabled, how can I expedite the replacement and updating of this?
3
You could import the private keys used into another wallet and spend the funds there. For instance, on Blockchain.info/wallet you simply do Advanced -> Import and import the private keys (which can be exported from Bitcoin-Qt's debug console).
2
I'll take a stab at this, from a very simplistic approach.
The best way to replace it is to pretend it didn't happen. Reset the blockchain in use, and continue as normal. The client (or bitcoind) will be unaware of the transaction, and the mistake transaction will eventually be disregarded by all nodes, invalidated, and discarded.
