Galaxy S5 G900V startup failure with inconsistent symptoms making troubleshooting difficult
I totally agree with all the symptoms you document. It reads like I would have wrote it. It seems many of us are suffering the same phone disease. Ultimately I ended up sending the phone to Samsung. They returned it unrepaired and stated the cost for repair  would be unjustifiable. Apparently, though, it can be repaired. I called Samsung, and, of course, talked to someone "who knows where". She did try and did say that an S7 root driver was installed into my S6. She said I could have done this while connected to my PC. Nope, the phone auto downloads updates. 
I don't understand why they cannot reflash the chip. I don't understand how I could be responsible for this and why I should have to purchase a new phone. 
Personally, I think Samsung sent the update without safeguarding that an older model would not auto download it. I think it's their fault and they are not taking responsibility. I didn't do this and now it's out of warranty, because of this unauthorised flash, irreparable due to cost. I'm screwed, we're screwed.

I'd like to see a class action law suit against Samsung and our providers. It smells of a conspiracy to make us upgrade $$$.

Gil