You described it perfectly how a heat pump dryer works.
As to the thermostats its not a thermostat as such its an NTC sensor. This is attached to the pip work of the condenser unit. If you remove the top and right hand side panel you will see the compressor from there. The NTC will be attached to the pipe work there. These are high resistance and are around 30K ohms from memory on these.
I don't think they will be the issue as when they fail it usually effects the running time and causes it to stop early then it should.
Also just a thought the dryer isn't located in an outside shed or something like that as these dryers don't like to work in cold tempertures as it messes the electronics and NTC sensors up. So if it is in a cold place just bring it into the house and let it warm up and see how it goes from there.
As to diagrams and service manuals im afraid there isn't any. As Beko do produce them but they are lock on the technical site which doesn't allow them to be downloaded or printed or saved. All we can do is view them on the site that's all.
Plus when these dryers fail on the system side as its a sealed unit we didn't replace them. Beko just replaced them straight away as they wasn't making the base units as for heat pump dryers most manufactores make it as a complete base unit were it came as a complete unit were as Beko didn't. Just looking at the parts list and it still isn't showing it as a complete base. But I can see the condenser unit and evaporator and compressor for sale on there own. Which unless you have the refridgration equipment you wont be able to replace them as it will need to be regassed once replaced.
But check the condenser fan and also check the condenser fins isn't blocked up as if these fail or blocked it can causes problems in the system side.