The code for the first two parts is below. Regarding your final inquiry, I personally like to separate the validations into individual sections. This makes it easier for future troubleshooting, especially if somebody other than you has to look at the code later.
'If cell T14 does not equal zero and cell T15 is blank, then have a messagebox appear.
If Range("T14").Value <> 0 And Trim(Range("T15").Value) = "" Then
MsgBox "CRITICAL: Please enter data into cell T15."
Exit Sub
End If
'If cell T14 equals 0, then return a blank value in cell T15.
If Range("T14").Value = 0 Then
Range("T15").Value = ""
End If
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