MacBook Pro, 15" (Early 2011), not booting I have a MacBook Pro 15" (Early 2011) model which suddenly is not booting correctly.

Symptoms:


Apple logo and loading bar appear.  
Loading bar loads part way, freezes for a while.
Screen flashes momentarily, then grey screen appears.
Grey screen stays forever.


Attempted troubleshooting:

Safe mode: symptoms unchanged
Single-user mode: symptoms unchanged
Verbose mode: symptoms unchanged
Clear NVRAM: Apple logo and loading bar appears, freezes, and stays that way forever.  After another reboot returns to original behavior.
Boot from USB memory (with MacOS Sierra install): boot menu appears, and allows me to select the USB memory stick for booting, after starting boot process from memory, returns to original behavior (note that I used same memory stick to load MacOS Sierra onto a Mac Mini with no problems)
Removed Hard Drive completely (and attempted to boot from USB memory): symptoms unchanged
Replaced internal memory: symptoms unchanged
Attempted boot with charger unplugged: symptoms unchanged

Question:

So, I'm thinking that, if it is not the  


Hard drive, or
Memory, or
Power Supply


...then it must be the motherboard that is bad.  Before I make that definitive conclusion, I'm wondering if there are any other ideas that I might try?

P.S.

On a side note that might be more relevant than I think, I noticed that this model is one of the models potentially implicated in the MacBook solder scandal (see https://www.apple.com/support/macbookpro-videoissues/ which lists the 15" Early 2011 model as one of the model lines with problems).  The symptoms don't exactly match what is listed there.  I haven't noticed any video problems (except for when I was changing out the memory: I got a couple of strange all-blue screens, but that could just be a result of poorly seated memory).  However, is it possible that a failing video adapter could cause problems at boot time?
Try Resetting the  SMC (if you haven't already, and see if that changes any of the mentioned behaviors)


Shut down the Mac.
Plug in the MagSafe or USB-C power adapter to a power source and to your Mac.
Using the built-in keyboard, press Shift-Control-Option on the left side of the keyboard, then press the power button at the same time.
Release all keys, then press the power button again to turn on your Mac.


The SMC or System Management Controller can require a reset and exhibit similar symptoms when malfunctioning. If this doesn't help, I can only suggest there may be hardware damage somewhere or possibly latent water damage side effects manifesting themselves.