Why does age come into most any question or answer on this site? I don't hear "I'm 72, what car should I buy" or "what is the best watch for someone in their eighties". How does it matter? Honestly confused.
The SB910 is a dated flash and has been discontinued. It may not be forward compatible with the D6. I suspect that is the case since it works with the D4. Not sure if you tried it in manual flash mode. If it fires in manual flash mode and not in TTL, than that is likely the issue.
The D6 manual you show has less than 300 pages. The is a reference manual on the Nikon website that is over 1100 pages. I would download and use the reference as it also has the bonus of being searchable.
On your last image of the books, noise grain aside, the image taken with the 5d miv is sharper. Noise reduction is a function of the camera and not the lens. R5 may be blurring the image a bit more in order to suppress the noise.
Sorry to hear about your experience but I don’t believe it is typical. I’ve been buying from B&H since the mid eighties. The rare times I’ve had issues, it was quickly fixed. They are tops in customer service. I would not hesitate to purchase anything from B&H. If they sold shoes, I would buy all my shoes from B&H.
The Neewer you mention is a strobe. The Godox is a speed light. They are not the same beast. If you are looking for a speed light you can’t go wrong with Godox.
I don’t think you have your facts straight. The Payboo card is issued by Synchrony bank and not B&H. Once you have a credit balance on the card they will send you a check if the balance is not used within a pay cycle. Call Synchrony to confirm.
Expensive relative to what? Something that is expensive to me may not be to you. And over time things change in in value often down in price. If you bought a Nikon D3S around 10 years ago, it would cost you around $6k. Now you can find a good used one for $1k. Was it expensive then and now it is cheap? Someone may still think $1k is expensive. And there are an unlimited number of examples one can point to. Just buy what you want and can afford. No other thought involved as far as I’m concerned. Cheers.