Not sure how you’d like to combine/display the date fields, so skipping that bit. But otherwise I think you’re just looking for the IF statement and maybe a demonstration of formulas within formulas? Something like:
IF( {Review Done}, "", {Days Until Due} )
You could also replace those {field references} with the code you’ve written if cutting down on the number of fields used is a high priority. For example:
IF( {Review Done}, "", DATEADD({Event Date},21, ‘days’) )
To combine those dates, make sure you format them properly (DATETIME_FORMAT) if they come out looking like gibberish. So your whole dateadd function would be inside of the datetime_format function. Other than that you’re probably just string concatenating. So "this date string" & " this other date"
becomes "this date string this other date"
.