I think the FSM will give testing parameters for individual sensors.
I think you can have faulty sensors that impact the EFI without them showing on a scan tool.
Trouble shooting 101
When you have a new problem after works been done, go back and recheck everything that's been done.
Be suspicious of everything new
1st in your list is engine rebuild.
24L/100km is too much for a fresh engine.
Even if you make the assumption a fresh engine is gonna be...
Look at what's been altered, or messed with recently.
Non standard injectors, particular off brand aftermarket parts would be high on my suspicion list.
Check everything associated with timing is working properly, and installed properly.
Crank position sensor
Cam and sensor
Cam shaft timing...
Sounds like an expensive fix for what is probably a simple problem. Either your mechanic ballsed something up, or has overlooked something
My 93 was getting about 17-18L/100km, a bit less in the city.
26/100 is what I'd expect for low range work or towing
38/100 would be flogging it...