One of the 2 cylinder banks not communication correctly;

There is only one intake air temp sensor, it's on the RHS intake, so the -40 on one bank makes sense.

 

Does it consistently throw a code for #5, or just the one time? Fuel supply issues can possibly throw codes on multiple cylinders on the same bank. These can be caused by overheating pump relays, bad pump motors, clogged pickups, or a clogged fuel filter. The black foam on the fuel pumps in these cars breaks down over time and then wreaks its havoc on the system. The wiring on the pumps is way undersized which causes the pumps to overheat and prematurely fail.

A few easier ideas you might try before things get a little more complicated:

Run Royal Purple injector cleaner through ~10 gallons of fuel and see if things improve.
Swap ECUs and see if the misfire shows on the same bank or follows the ECU.
Swap the #5 injector with an injector on the other bank and see if the misfire follows the injector.

Do you happen to have a set of oem plug wires you can use for testing?

 

Good call, swapping the coil might help, though they will perform less effectively when warm so there may be something else. Do you happen to know if the intake manifold gasket has been replaced? A vacuum leak test when the car is cold might be a good idea. That is an issue that can go away when it warms up. I'll keep thinking as well.

 

Just be aware that the commonly available pdf file of the 550 wiring diagrams mis-titles the left and right banks.

Note that there are actually two coolant temperature sensors. However, the left coolant temp sensor does not talk to its respective ECU, but to the HVAC ECU which I believe sends the data on to the instrument panel water temperature gauge.

Assuming both bank ECUs need coolant temperature, it must then be up to the Right ECU to send right coolant temperature sensor data to the Left ECU. I have read on the forum that the engine needs to be running for inter-ECU communications to be established. There is a wiring harness which runs between the two ECUs (with a connector dividing the harness in two). It's not clear if the temperature information is sent on an individual wire or via a databus. The dividing connector has 12 pins and is located behind the engine (not easy to get to)

Anyway, as this is a USA car, I assume it has an OBD port and diagnostics should be easier.

 

Has your car had the fuel pumps addressed? If not I would check for fuel pressure at the fuel rail of the bank that is not firing.

 

Sorry, getting my systems mixed up. There is a separate sensor for gauge water temperature.

Wiring diagram:

https://www.dropbox.com/t/2xMbAYThAeb82iLp