"The money must sit in the account for six months"
This one has a real ancestor, which is why it survives. IRCC requires the bank letter to state the average balance for the past six months of each account. That is a rule about what the letter has to say, not a period the money has to serve. Somebody read it as a holding requirement and the misreading became the standard advice.
A recent deposit does not disqualify you. It has to be money you can legally access, and you should be able to say where it came from.