A: There is no easy way to do this, you will need to change every transaction reference for account A to account B. Use the "Find Transactions" feature and search for splits where Account = account A. This will list all of them in a single journal-style register window where you can actually edit the accounts. Change the transfer account in each from A to B. As you edit the transactions they will be removed from the register. When the register is empty, account A will be empty and you can remove it.
A: This is available from the menu: File -> Export -> Accounts
A: Enter a search criteria of matches regex, and place a single dot "." in the text field area. Then, click Find. The regular expression "." means to match anything.
A: You record the transaction on the date you write the check or initiate the transaction. When it "clears" the bank, you can click in the "Reconciled" field to "clear" the transaction (change the "n"on-reconciled to "c"leared).