They can (and probably will) make money on the after dinner circuit, in the way that eg Boris Johnson does. I don't know how much money, but it's how a lot of famous people make extra cash, and would fit with the idea of being thought leaders.
If I was advising the Royal family, I'd recommend waiting until after the book is out before deciding whether or not to meet with H&M. There's too much risk of something else being in the book to do anything beforehand. On balance, I think it probably would be better for all concerned if H&M were at the coronation, but (for either side) not at any cost.
On Elizabeth II, it's incredibly difficult to know what is an accurate depiction of her as she was really very good at not giving anything much away. She definitely had advisors, and it is widely believed that towards the end of her life, Charles had more of a say in her decisions than previously (given he would have to live with them, not her). Harry's interpretation of events is possible, but as others have said it's perhaps more likely that she made the decision and/or fundamentally agreed with it.
Past evidence (eg her sister) is that she has a lot of sympathy for family members who want to pursue their own happiness and will offer options. But not at the expense of what she considers is important the institution (which has changed over time eg in relation to divorce).