[stuhood]

1) Yes and no... the QT client has default/"suggested" transaction fees, but unless a block is completely full you're not likely to see any difference in processing time if you exclude a fee entirely [a]

2) As of recently [b], yes... you can add a small amount of junk data

3) When you send a transaction using input address(es) that don't add up to exactly the desired amount, there will be some "change", which needs to go to a new address. Thus, some of your change comes back to you at a new address.

4) Yes, that's possible. But you probably don't want to do that, since it leaks information [c]

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[a] subject to some limitations: https://en.bitcoin.it/wiki/Transaction_fees#Sending

[b] https://bitcoinfoundation.org/blog/?p=290

[c] https://medium.com/p/7f95a386692f