Article II, Section 2 of the US Constitution is clear that treaties require 2/3 of Senators voting in the affirmative to enact a treaty.
No, his "compound" was not on Ambergris Caye. He initially lived there, but he then built his "compound" on the mainland. Google the lengthy Wired Magazine article about him where you will find this sentence "He began spending more time in Orange Walk, a town of about 13,000 people that was 5 miles from his compound." and then use Google Maps to locate Orange Walk on the mainland.
McAfee's "compound" was near Orange Walk Town, on the mainland, which is nowhere near Ambergris Caye. One wonders how many other "facts" in this story are completely false.