At the beginning of the "Marvel Era", Stan Lee's "financial security" or "job security" wasn't worth anything, as the company was failing. This is a bit of history that both Kirby and Lee agreed on, that as far as they knew, if Fantastic Four wasn't a success, the company was folding. 

Once the comics started selling, both Lee and Kirby had de-facto job security. Lee had it formally, but Kirby was handed more books to draw than any written contract could possibly have guaranteed him ( No lawyer would allow an editor to give an employee a contract guaranteeing them 400+ percent employment. And Kirby was doing more than 4 books a month at his peak).

At the point towards the end of the 60s that Lee finally managed to get authorization for creating an Art Director position for Kirby (something he had pushed for for several years, apparently), that if history is any judge, would have given him financial security for the rest of his life, Kirby turned it down.  

John Romita SR, who finally ended up in that position, didn't do so badly by it, did he?

So throughout his career at Marvel, Kirby did have financial security, just not on paper.  At the point when he himself wanted to step back a bit and do more plotting and development of stuff for other artists (which was the original plan for New Gods, that some of the series would be drawn by others),  he was given the opportunity to do just that as Art Director at Marvel.

And then he turned it down.

A lot of Kirby's gripes about getting less than Lee, less credit, less respect, less money, tie back to that choice. Now, as I understand it there were family health reasons why they wanted to move to California, but it still remains that he was given opportunities at Marvel that could have remedied all the problems with lack of credit or financial rewards.

Before getting too far into accusing Marvel of "using" Kirby and then "throwing him away", we should remember that he walked away when they offered him everything he said he wanted.

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Which is how it worked for everybody, unless writers and artists worked out special deals between themselves. I did considerably more than just drawing on UNCANNY X-MEN, but drawing was all I got paid for. Since I got paid more than twice as much as Chris, that didn't seem such a big deal.

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An industry that has been around for 20 or 30 years is considered "fledgling"?

My question is, for those who understand the law. What do the Kirby heirs have to prove and what is Marvel required to prove in regards to this? Who is required to prove their case? Those that are suing or the defendants?   

To paraphrase my own previous comment, this comes down to Kirby (at one time) and his heirs (presently) saying "You mean the rules that apply to everybody else (and that I applied to my employees) apply to ME?"