It's not fitted right no, but neither of the mistakes on Installation would cause the vacuum created that has imploded the cylinder.
There's integral non return valves on both hot and cold draw offs on the combination valves.
One could argue that the absence of an anti-vacuum valve could be a cause... But it's not compulsory - only recommended to avoid scenarios exactly like his where the specified drain down procedure isn't followed.
Best to have a Lever valve on outlet and another on balanced cold and utilise those for tap changes etc.
It's a tough one as your not g3 your technically not competent to have had knowledge of this, but on the flip side, it is in the User instructions regarding correct drain down procedure. So the responsibility would still fall down to yourself.
Boilermate thermal stores behave exactly the same way if any attempt to bung them is tried.
Installation should still be corrected regarding the pipe layout/expansion vessel location. But as it stands the 2 components would still be operating in the same way as if they are now and wouldn't have contributed to this scenario.
