The Articles of Incorporation are going to be the key legal document. Still, the Founding Agreement is important to demonstrate the original intentions and motivations of the parties. That builds the foundation for the case that something definitively caused Altman to steer the company in a different direction. I don't believe it's unfair to say Altman is steering; it seems like the Altman firing was a strategy to draw out the anti-Microsoft board members, who, once identified, were easily removed once Altman was reinstated. If Altman wasn't steering, then there's no reason he would have been rehired after he was fired.
Subject to limits on specific kinds of contracts that must be reduced to writing, all US jurisdictions (not just some states) recognize oral contracts provided that the basic requirements of a contract (offer, acceptance, consideration, etc.) are present.
Yes, except for the narrow situations where writing is legally required for a contract, the point of a written contract document is not that it is necessary to create a contract but that it is valuable in the event of a dispute as evidence of what the parties actually agreed to.
Determining that an oral agreement existed and what the terms were is an evidence problem.