I was a bit short on time to proceed with testing these last days, but now I’ve been able to realise the next steps.
I’ve used the needle-toothpick leverage method you had suggested. Thanks for the detailed description. It worked well. I had started with the IC that warmed up the most (IC502) and lifted its VCC leg. I then measured resistance VCC-Ground and it was a bit disappointing: it just had gone up by some Ohms. So, I went on also lifting the VCC legs from the other 3 ICs (always measuring in between if there had been a major change in resistance after having desoldered a contact). After having disconnected the 4 ICs (IC502, IC503, IC506 and IC504) from the VCC circuit, the resistance between VCC and Ground had gone up from 4 Ω to 19 Ω. According to Ohm's law (I = U/R), this would mean that there still would be (5V/19Ω =) 260 mA of current flowing through the VCC circuit, what seems too high. So, I again connected the voltage source and applied a voltage of 3.5 V to the VCC circuit to measure which component was heating up the most. It was IC518, of which I then desoldered the VCC leg also. That made the VCC-Ground resistance climb to 30 Ω. That would correspond to a 60 mA current through the VCC circuit at 5V. Again, I applied the 3.5 V to the VCC circuit and the components now warming up the most (but way less then the other components before) are the IC704 and IC717.
These were the facts, now starts the interpretation:
The observed temperatures of IC704 and IC717 aren’t that much above the temperature of the rest of the board and for IC704 (a TJA1050 from NXP (was Philips Semiconductors before)) it seems normal that it dissipates some heat according to its datasheet
(bottom of page 5). The datasheet also says that in dominant state it sucks 50 mA of supply current (top of page 6), what could explain the biggest part of the 60 mA flowing through the VCC circuit (at 5V).
Concerning IC717 (marked “VHC, 14-S, B1 14”, if I correctly identified it, it’s a 74VHC14FT from Toshiba, out of the “74” family like IC502, IC503, IC506, IC504 and IC518 but with a Schmitt Inverter logic) it’s probably more unusual that it heats up, as the datasheet
says that these chips have low power dissipation and a very small current between VCC and Ground (quiescent supply current of 2 µA).
As all the mentioned Toshiba ICs were exposed to the same abnormality, it doesn’t seem unlikely that all of them were damaged in a similar way. Maybe some a little bit more, some a little bit less.
Due to these last findings, I’d say that a short between 5V layer and ground layer seems unlikely.
The difficulty I’m now facing is to know if this obtained resistance of 30 Ω between 5V-VCC and Ground is the resistance we should observe in this situation, or if it should be higher. That would determine if all damaged parts have now been disconnected from the circuit, so the repair could be limited to these parts, or if the search for other potential shorts needs to be continued.