VDDEXT is designed to be used off-board of the ECU. However, VDDEXT is not protected against high voltages (e.g. caused by a short circuit to the battery supply voltage). The TLE986x and TLE987x device families also offer an internal protection against overcurrent.
For the TLE987x device family, there are internal overload, overvoltage, undervoltage and overcurrent detectors. If enabled, all of them will trigger an interruption in case of such a detection. Similar detectors also exists for other device families.
Yes. If VDDEXT is externally connected to the VAREF pin, it is important to deactivate the internal 5V-Varef LDO, before activating VDDEXT. Otherwise 2 LDOs will try to supply each other, which will cause malfunction.