It doesn't take that long to put some safeguards.
Here is the thing: WHENEVER you say "you have to be VERY careful" or blame someone for making a mistake, that's a place you need to put more checks. The computer should check routine things, not people.
"The problem I'm having is where do you start? How do you simulate the data flowing through live in the test / dev enviornment."
I am guessing you use MySQL so simply set up MYSQL REPLICATION and test all your stuff on the replica database first. Actually you should periodically dump the replica and test on a snapshot, so you can mess up the snapshot.
Main db -> Replica -> Snapshot.