Unfortunately Endevor always get the blame, right up to the point when I ask "Oh, you did your Final (or User Acceptance Testing etc.) in Stage 'x' before you implemented didn't you?", pointing out that proper testing should have identified the code regression. Stage 'x' being the stage were the paths/code merge.
You might also want to point out that Endevor did the right thing and alerted the application team to the problem via the SYNC error. The fact that the application team did not react correctly is not Endevor's fault.
As to education, Change Control, Configuration Item Control and Standard Testing practices are not Endevor training but should be part of every Application Team skill set. I was doing these over 20 years ago without Endevor, and while Endevor makes these a lot easier it should not remove responsibility from the Application Team. Perhaps you should suggest ITIL training etc.
You may also want to review who can perform OVERRIDE SIGNOUT and restrict this access as well.
It seems many groups want Parallel development, thinking it'll be easier, without understanding the risks and the costs involved. We get the occasional request for parallel environments but most requests are withdrawn after we carefully explain the risks, management and overheads and we make 'retrofitting' and re-testing of code a mandatory task of the Application Team. The few that still go ahead are the ones that really 'need ' (rather than 'want') parallel development and I can tell you they spent many hours on analysis and retrofitting code (some use PDM and some don't) and that it's a significant part of their workload.
For some teams, we have introduced 6 stage environments and this seems to satisfy their need for multiple releases but keeps the release order in place (as well as SIGNIN/SIGNOUT processing).
Oh, and your site should have a "Configuration Management Plan" and Endevor should be configured to support that plan. If you have one then I suggest you become very familiar with it, as It should provide much needed ammunition. If you don't have one then you should ask for one to be provided.
Good luck.