To fix the first part, use two different triggered smart campaigns.
First camapign will be Triggered off "Data Value Changes", Field is "Custom Field", and use the Previous Value and New Value constraints. Previous Value could be something like "Is Empty" or "Is Not" C, and the New Value=C.
The second campaign will also use the "Data Value Changes" trigger, use "Previous Value" constraint, with "is C", and use "New Value" constraint "IS", click the green +/circle, and add A, B, and D.
To fix the last paragraph, in flow, use the "Send Alert" Action, select the name of the Alert Email in the drop down, then select Send to : "NONE". Then add your email address to "To Other Emails:" section. You'll be the only person to get the mesage.