Sorry for the delay in responding to your posting.
As far as I can understand your problem is as follows:-
- Sound and Vision go completely
- You cannot turn off the TV with the remote
It sounds like you have a heat related power supply fault where a component is failing when it gets hot, unfortunately B&O's service policy these days is not to fix to component level meaning the dealer has a kit of all the assembly parts, that's why your dealer has told you that he will have to change the chassis which will not be cheap.
Unless you are a technical person with knowledge of electronics/TV you will need to get the B&O dealer to fix your problem.