It will be fine if isolated whilst away i always turn off mains water when im on holiday iffact some home insurance policies state you must. Take some pictures you may not need a new cylinder
Can you turn preheat off by turning hw temp down on that model? If you can monitor the led then, i was talking about an led on the sensor you can see when the case is taken off that gives a clearer indication
You could run the shower and observe what the hall effect sensor led is doing, you will know if you are getting a good flow through the boiler or the sensors goosed, could even be a mucky plate does it only happen on the shower
The heat exchanger may be under 10 year warranty, check your paperwork/speak to Worcester to find out. If not covered a fixed price repair from somebody would be your best bet or depending on boiler age then replace it.
Turn power off to boiler overnight and put a bag over condensate outlet to see if it collects water if you have no leak from prv, are you using heating at the minute?