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The usual U1950/U6100 traces/resistors, J6100, crap under SMC.

Sounds like a power supply issue to me.

Picture pease? Apple never used red edge bonding for the GPUs.

A beep is totally different from a chime!

Need UBRT cause the other one cannot unseal the newer BQ chipset. All 100% hopeless, don't waste time.

Very likely it is just an LCD problem. Get some cracked test assemblies and make sure that you use the correct year. 2015 machines do not work with 2014 LCD assemblies!

U7200 needs to be told from the CPU to turn on. The problem is pretty much never with U7200 and more likely with the CPU. Just try another BIOS and if that does not help I would call it a night with this one.

15V is normal. There were a few different audio/trackpad cables so it sucks if you do not have the exact number. Did you not plug in audio cable upside down?

You can try, what is there to loose?

Check with USB mouse if it lights up. It could be anything at this point especially if it was messed with. I do not believe it "just" happened. Corrupted BIOS is also an option.

No it is a metallurgical problem. Copper that is cold deformed has internal stress in it and without annealing it it will in some cases start to crack to releive the stresses. Copper/brass are very prone to this problem and it is known. It is also referred to as stress-corrosion, common anywhere...

Measure resistance from PPBUS to all coils starting with L9330. I bet there is a 0 Ohm short there.

Echo Chime is usually RAM or CPU issue. Is there crap under the RAM or CPU?

G2_EN present? Voltage on center pin of magsafe with adapter connected?

Run ASD EFI version first. Was is already messed with? Especially GPU area.

Came with no power and powers on now? What was done? Need to ask customer for history!

I cleaned the ME region and took a known good BIOS and pasted your serial number into that. Then the checksum of the Fsys block that contains the serial number was corrected. I can write a book about this or you can just buy the Medusa2 at cmizapper.com and that will do this for you.

Exact values? History?....

If Vcore is not coming on then the CPU can never process the BIOS. Vcore should at least do something before you should worry about the BIOS.

Run ASD first. Is the piece of tape still on the airport connector? The metal sponge on the cable can short out the pins.