Selling shares for money to donate to charity is actually a tax mistake. I've done it and regret it. Just donate the shares to the charity and let them cash it out.

Surely the CEO has a tax advisor to advise... so this leads me to believe that very little of it will "go to the charities"

Pre-planned sales for execs are often buybacks or are sold to selected investors/shareholders.

Companies often impose limits on how many shares execs and employees in general can hold at any give. time, and most regulatory/legal frameworks put caps too.

In the UK for example when employees own more than 5% share in a company they are no longer treated as regular employees.