Hi....welcome to the Forum
Sorry to hear you've had so many issues with the Astra.
I would say that as it has had work already done on the engine cooling system then that's clearly where the fault is.
Engine coolant does just that 'cools the engine' and if any components fail or are lacking in an ability to achieve a regulated operating temperature then this is where the fault lies.
If the engine is driven to destruction then this clearly is as a result of running out of coolant (leaks, etc) or it is something else, like a head gasket which could have been the original cause of the problem.
As regards the piston rings being fried, it would only be evident upon tearing the engine down to examine it properly, or if you are very lucky, a compression test and a leak down test would reveal whether there is a lack of compression in the cylinders.
I would advise taking it to another garage or have a mobile mechanic come round to diagnose it
Let us know how you get on with it
Cheers Trevor