Of course when you disconnect the battery, you lose the computer sensor settings that have been learned, and that may create some issues.
Iāve not had much problem disconnecting the battery with regard to loosing settings. The radio stations presets are still there. I think Iāll loose some DIC items like gas mileage since the last reset. Disconnecting the pos cable is kind of a pain. Do you think if the dealer canāt diagnose they could install a battery positive off switch, preferably in the cabin rather than under the hood? 
Has the alternator been replaced ?
I have seen alternators with a bad diode still properly charge a battery while running, but drain the battery in a couple days of sitting.
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I donāt think an in-car battery cutoff would be advisable due to limited space and high amperage, heavy duty cables.
It would be possible to mount an old fender mount Ford starter solenoid under the hood, splice the positive cable into that, and then use 2 small wires to an in-car switch to activate the solenoid and which would then allow current flow.
The problem there is that considering this dealer has had the car 3 months with no clue I canāt say that Iād trust them to perform this reasonably simple task.
Personally, I feel it should be fixed right rather than cobble something together.
Just curious, but is there any indication from this dealer about what if anything they are going to charge you for this debacle? IMO, at this point they should not be charging you anything.
If they canāt figure it out they should have thrown in the towel the first week and advised you to take the car to someone else.
I paid $1400 for the Fuse box replacement and fixing the sun roof tube leakage. I feel obligated to hang in there while they are providing a loaner. As suggested earlier maybe a generous buy out is in order.
Did your shop tell you how many mA the phantom current drain is?
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Battery Drain Update: Around the 2nd week of May dealer said they thought the drain was coming from the instrument cluster which was on back order They asked me to turn in the Colorado as the miles were approaching 4K. They provided me an real nice18 Tahoe with 383 miles. Got a call today the Enclave was ready to pick up. I asked them to double check the new issues I had after the first fix back in March, let the vehicle sit a few days to re-test the drain. Hoping to have the Tahoe at least over the weekend before turning in.
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Thatās quite an ordeal youāve been through there OP. Hopefully the problem on your Buick is now fixed. This is in part the result of all the modern gadgetry car buyers demand. Thereās nothing simple under the dashboard these days.
Hopefully it is fixed. I probably wonāt hear from the dealer until next week which give me a few more days to enjoy the Tahoe.
I have a 46 year old Ford truck. The dashboard ā containing a few instrument panel gauges, mechanical switches, and the push/pull heater control ā is held on with bolts and screws, the heads clearly visible. Removing the dashboard wouldnāt be that hard. On my 27 year old Corolla, with more involved gadgetry and hidden fasteners, it would be a much bigger job. I presume your Buick is a much newer model, so I canāt even imagine how difficult it would be. Be happy thatās an SEP: somebodyās elseās problem 
If he was being charged for a āloanerā it would be a ārentalā.
Donāt understand, what is magical about this number of miles?
IDK but was told by service they prefer to keep the loaner mileage under 3500.
Because the car will likely be sold as a demonstrator, and if you get over 4-5000 miles, people donāt see them as demonstrators. Plus it wouldnāt surprise me if there were restrictions on the miles that can be driven with dealer plates and not registered.
The āgoodā old FOMOCO starter relay (What Ford called them). With 2 Galaxies, 4 Mustangs, 1 Lincoln, and a Crown Victoria I learned early on to keep a new one in the box, wrench and nut driver in the glove box. I could eventually change one in 5 minutes! I donāt recall the Buick dealership I worked for in the mid 1970s having a dedicated automotive electronics specialist. The 2 tune-up guys had their diagnostic machine that could test charging systems so they were tasked with that. Of course automotive electronics were much simpler then but getting more complicated.
The Ford starter relay was a great piece of electrical engineering. Most of the guys at the dirt track here run small block Chevys. They disconnect the stock Chevy solenoid and run a cable from the starter motor to a Ford relay mounted on the fender. It makes things easier in the event of a no-start when hop laps are about to begin.
Many years ago while working for VW, we used to add a Ford relay to many of the old VW 6 volt Beetles. The voltage drop was severe enough with the wiring going from under the rear seat to the ignition switch and then back to the starter at the rear of the car that a no-start was not uncommon.
The Ford relay with a few wiring additions cut that distance way down and eliminated the problem.