Sounds like the first step you have is to sell to the team the need to invest in new software methodology. Per your statement, there is no concensus in the team, and you'll need it to be able to plow ahead with a slow "upgrade" of the code.

I would (if I can) personally take the hard lessons learned, and introduce each of the key concepts that you want to as the solution to the problem in the software industry.

E.g. two developers had different copies and ended up deploying a hybrid untested release -> Introduce version control, branching, and testing.

Somebody removed a few lines of code that they didn't understand and caused an outage -> introduce DDD.

If the hard lessons aren't being shared with you in sufficient detail, then just show your own examples of how things went wrong when this discipline wasn't adhered to.