ECM replacement?

If you find something odd, do let me know and we'll figure it out. I'm assuming you're in a hot state being 85 yesterday. I'm in NC, so I understand you really need your A/C, I'll continue to do what I can from here. I'll do a little extra research and see if anyone else is having a similar issue.

Okay, I'm going to have you check the A/C high pressure sensor and the ambient temperature sensor, I'm not comfortable leaving situations unresolved. The A/C pressure sensor is on the high pressure line, which will be the smaller of the 2 lines, plus I believe there's only one line pressure sensor. First picture is the plug for the sensor. Pin 1 is ground going to the ECM. Pin you should have a solid 5v all the time. Pin 3 is your pressure signal wire 0-5volts. While the car is running and since the compressor is not coming on, low pressure will be a lower voltage signal and high pressure will be more towards 5 volts. I usually back probe these type connectors without unplugging them. I use a T-pin to back probe connectors. Just be careful not to damage it. Keep the A/C turned on incase the compressor decides it wants to come on. The next thing I want you to do is a wiggle test of the harness around that fuse box, any contact points and the harness running to the ECM. If doing the wiggle test, the compressor turns on, then you're very close to the wiring issue. Wiggle the connector on the compressor as well just to be sure. Just watch your hands by any moving belts or components. Let's see if any of the wiggle test has any effect. The ambient temperature sensor is the one going to the radio. If you unplug it one wire will be 5 volts, the other is ground to ECM. It's located on the front grille, it's only 2 wire little sensor.

The 5 volt reference is only with the key on. The ECM sends out the reference voltage to many sensors on the car. So you haven't found anything yet? Did you locate the ECM at least I hope? They put it in different places on these cars depending on the engine size. I'll dig up some more diagrams. There's other temperature sensors all throughout the car, plus the sun sensors. I'd really like to help you find this problem. I think I have you most of the diagrams already. Make sure your coolant is full as well, the engine coolant temperature sensor plays a roll in the ac system too. If the engine is too hot, over 257f it won't turn the compressor on.
What readings did you get on the pressure sensor? High or low on the signal wire? And do you know where the HVAC control module is?
This is interesting, "when the A/C switch is pressed, the HVAC control module sends a serial data message to the ECM for an A/C request. Which in turn grounds the A/C relay." That's right from all data. I'm willing to bet there's a code stored in the HVAC module. Not all scan tools can read every module on the car. Like the ones at AutoZone can probably only read 4 modules, ECM, TCM, ABS, etc. Some interesting reading here for you too on intermittent wiring issues. I'd recheck the battery connections too since you said someone hadn't tightened them all the way.

Let me know what you find, there's a few things coming to mind here, if you haven't found any problem areas yet. This is becoming a very long diagnoses.