The use of the BC817 type transistor, instead of the 2X LL4148 diodes, offers a closer bit of control over the setting of the threshold where the current limit kicks in. This advantage is offered due to the current gain in the transistor.

Yes, Michael, this IS a relevant improve! Makes the circuit to work more reliable with manufacturing tolerances.

I do also like the way you connected the BD135 to the cooling plate. The bigger the heat capacitiy, the longer the diagnosis intervals can be made. This is especially useful, when so many different floors have to be controlled.
The controlling of lamps could be made in such a way, that all the time rows are sent into the HC595s and are back checked by reading the according HCT597s. This would minimize an eventual short circuit duration.

An additional word to the 10k resistors at inputs of HCT597. The according 5V supply must be able to tolerate the injected current from the +12V lamp supply! Maybe the resistors can be increased to about 100kOhm...

Kai