2013 Yukon Denali XL 1500 6.2 overcharging

Not a full time mechanic, but work on all my stuff excluding transmissions and was a parts changer 
2013 Denali is cutting my tail! This is the father in law's car, came to me with a belt squeak and loosing coolant.
Summarized History:
Water pump, Radiator, heater hose, Tee fitting later no leaks under 20lbs of pressure.
Drive the 1st time 30F outside get the running hot turn off AC pop up with MIL.
Finger in coolant test and is not even hot,
Replace the ECT, still having issue
Vacuum the coolant system, refill and good.
Side note start
While troubleshooting and pulling codes noticed the system was charging 15+v and after research found it was normal for this vehicle or thought to be.
Also notice only a few min of ignition on not running, message of low battery to start engine.
Rigged a digital voltmeter for the cigarette lighter/12v outlet.
Coolant system still good.
Removed the ESM and put it back.
Some of this testing did without driving lights, wipers, AC or rear defrost on.
Found a short on a wire connecting to under hood fuse box and repaired
Side not stop
Still have the squeaking issue to resolve.....notice belt squeak is after sitting for 12+hrs.
Replace belt and tensioner and squeak went away only to return the next day.
Turns out the battery is over 5yrs old and doesn't hold it's charge as it should but also parts of the onboard systems are not reading the same.
Voltmeter at the battery and the voltmeter in the 12v outlet are within .2v and scan tool within .5v of them both.
The dash voltmeter is about 1.2v off and know they are not 100%
Battery changed and still the same thing minus how quick the dash notice bat low start engine appears
Cold start, alt signal unplugged, no squeak and 13.8v (as it should default to).
Replace the alternator still same thing.
Replace the battery current sensor.
Find ScannerDanner and multiple videos ect 
Go to return the alternator and put the original back on BUT is smells burnt!
Leave the new one and start digging in.
Found the negative terminal was creating resistance and replaced just the terminal via cut/replace.
Started doing some tracing and all the sensors I tested had a 5v ref but found the scan tool showed that ignition voltages were different,
Found that the 5v ref via circuit 530 from BCM to ignition had a good 5v reference with the switch off and system awake and when the switch is in the first click/acc of the switch.
When in the run position it drops to 4v, while cranking goes up a little and with engine running 4v which is in line with the difference I am seeing on the scan tool.
Start probing the actual ignition switch and find the same results, replace it with same results. (killed my scan tool cause I acidentaly shorted across the switch, blew the DLIS fuse and kill the tool connected via OBDII DLC)
Using the Autel AL539b and another cheap oscope found the PWM signal is not 120hz but more like 96hz
The signal seems to respond the same when tested at BCM, Batt Current Sensor and at altenator
Found using the xtool old and new the Electronic Suspension Module becomes unresponsive more times than providing data
Found that it seems there is a ghost frequency found at the under hood fuse box which I think is related probing the side of some of the fuse connector it shows a small amount of voltage with a signal
Found using the xtool to set the duty cycle of the alternator it responds and will stay set for a while then slowly starts to climb.
The xtool also shows the desired voltage is less than the current voltage
Know I have did way more than listed here and probably forgot to list some of the important testing I have already done.
Help me wise ones!!!!!! 

The dash voltmeter is about 1.2v off

I would investigate the power feeds to the cluster first, then the bcm, for a voltage drop.

Battery Voltage Gauge OperationThe IPC displays the system voltage as received from the BCM over the GMLAN serial data circuit. If there is no communication with the BCM then the gauge will indicate minimum.This vehicle is equipped with a regulated voltage control (RVC) system. This will cause the voltmeter to fluctuate between 12 and 14 volts, as opposed to non-regulated systems which usually maintain a more consistent reading of 14 volts. This fluctuation with the RVC system is normal system operation and NO repairs should be attempted.

This IPC fuse 46 is interesting. If you have 15+ volts at the battery you should have this same voltage at that fuse. See attachments at bottom.

 

Morning all, I have not had time to get back to the Yukon yet. I will update when I get to it

Hope all is well!
Finally had a minute to test the following, Each was tested using a homemade extension/lead from the ground. This is to limit the fluctuation by not touching the metal right at each location. These were all done with the vehicle having sit for about 2 weeks without anything being touched.
The system had just got woken by me opening the door and can see by the end of the test it was going back into sleep by the voltage increase.
1st picture is + to - across the battery using the extension for ground.
 
2nd is to batt volt on alternator
 
3rd is fusebox hot leg
 
4th IPC 46 Fuse
 
5th is the homemade voltmeter in the cigarette lighter
 
6th is measurement of the back side of the voltmeter just to keep everything in perspective (also system is a sleep and battery is recovering)
 
 

Running  + to - across the battery using the extension for ground.
 

Running batt volt on alternator
 

Running fusebox hot leg
 

Running Fuse 46 IPC
 

Running homemade voltmeter in the cigarette lighter
 

Running  Dash
 

Acces Dash
 

Acc homemade vmeter 

Before going knee deep into this i think the easiest thing to check is to see what the BCM and cluster "thinks" the system voltage is at.

   

 

Anywhere the scan tool is looking for 12v that i have found is inline with the multimeter. The only exception is when I go into the module to change the charging controls. Will work on getting pictures uploaded, hopefully on their on the new scan tool not the busted one.

Had to get the data/picutres again as screenshots were on the tool I blew up.
Hope to upload and explain this weekend

Have another round of pics to explain and hopefully answer the questions.
Using Xtool D8 for the below pictures.
Have captured several images but only posting the ones I think stand out or to answer about questions comments to prevent pic overload. 


 
 
 
 

Hoping to get your thoughts on the data provided

Have you seen anywhere a "desired voltage" PID?

I have