... the truth is, the book was well advertised and had a long delay, there was plenty of time for the Buscema's and/or their agents to inform themselves on what books containing Buscema works were being released, no-one denied them of that action and if they choose not to do it then it is their decision
Nonsense. We knew about it because we collect comics and follow the releases of collected editions. If you don't collect comics or collected editions, you're likely not scouring Diamond Previews or websites like this. You would have no way of knowing. Heck, I bet there are collectors who only deal with Marvel and DC and never look at what a smaller company like Hermes is doing.

My guess is the Buscema family does not follow comics collecting and keep current with what is coming out. Even the younger Ms. Buscema who works in the field might just work in the field but not follow every publication of every publisher. i honestly think few in the field do that, if any.

My further guess is that none of them knew, until someone approached them with the book and showed it to them (thinking they knew about it).

To assume because you know about it, that everyone knows about it is a fallacy.