Document everything.
Get signatures for documents.
Have a requirements change control process that is punative. Home builder have this part down. Make them agree to this upfront.
Deliver and keep signed documents handy.
charge more. the more trouble they give you, the more you have to charge.
You FIRE them !
Jerks, like the CTO you described, do not become more cooperative over time. It is possible that he is trying impress the investors and co-founders. Sadly for you, there is no way to win in these situations. So it is best to move on.
document everything and get it signed.
reference it when asked.
I don't see a difficult client, I see a consultant who didn't ask the right questions before taking the job and isn't seeing things from their client's point of view now that they have it.
Why don't they show up to standup? Why aren't your emails a priority to them? Why was a demo to the CTO necessary or desired? Why did they leave you out? Why was that flow important to them? Why did they discuss something out-of-scope?
I only see your perspective. If you don't know the answers to these questions, why not?
Advice isn't going to help; you need a relationship manager. You probably need to bring someone in to get face time with this person, and handle them on an ongoing basis, again probably in person every time. It's going to get expensive. You're probably not going to make as much money on this contract as you expected as a result. This isn't a technical problem to be learned from and fixed; this is an interpersonal problem with someone who works and thinks differently than you, who is also unfortunately holding the purse strings.
The most valuable thing I learned working in consulting wasn't how to be a better designer or developer, it was how to handle relationships. I learned that the next time I went out on my own, I'd hire a business and relationship manager, and I would absolutely pay them 50% or even 60% of my earnings, because that is the "real" work of consulting.
A relationship manager will give you a list of signs that you shouldn't have taken this job in the first place. A relationship manager would have specified that this person's attendance in standup is either mandatory (and therefore in the contract) or not required (which means they wouldn't have even been invited). A relationship manager would probably have established up front how this person likes to work, and that probably means in-person, and would never have sent an email, and instead had lots of meetings. A relationship manager would have made sure you kept control of the demonstrations, so off-message presentations couldn't have happened, and would have made sure that out-of-scope things either didn't come up, or were quickly addressed by understanding why they came up.
Your takeaway from this will be how to better vet potential clients, so you can either not take their money, or add in the cost of relationship management to their contract. You'll also learn to write better contracts, that firmly establish the participation required, including penalties up to and including the termination of the contract with full payment and no deliverable, should the client violate their expected level of participation.