Have you tried the navcores on an empty tomtom? Back up your map folder to your computer and quickformat your tomtom. Then extract the SE Navcore to it and see if it restarts. If it does start up with "no maps found", you continue adding the map. If it does not start up, you need to update the bootloader to 5.5*** after all.
Download it from Tomtom.com so you can be sure of an untampered file:
Extract the cab file from this download and use ONLY the file called system. Throw the rest away because it is not an SE navcore and will not run on your device.
Copy the file called system to your tomtom and let it reboot. Make sure that the battery is fully charged before you do this. Once it has rebooted, it will show bootloader 5.5128 and you should be fine with the navcore SE 8.562.