As to the following:
• Only include Void periods if break options are exercised, i.e. only include void periods if column D = “Yes”, if not then no void periods should apply.
• Rent is currently pulling into the model before the start date specified in column C – this should only commence on or after the date specified in column C.
Try: =IF(K$2<$C4,"",IF(AND($D4="Yes",OR(K$2>$F4,K$2>$E4),OR(K$2<=EOMONTH($F4,$J4*3),K$2<=EOMONTH($E4,$J4*3))),"Void", $G4/4 ))
As to the rest please manually include examples of the output you expect in each possible scenario and, unless it is obvious, please include explanations of the expected output.
If the values in column D are pulling from the "Data Input & Output" sheet then data validation for the cells in column D is not needed.
Let us know if you have any questions.