It seems to me that there are two issues here: the question and the direction the thread has taken.
With the question, I see the problem as stemming from there being one continent and one country with the name 'America', so I see 'Americas' as referring to regions of the continent, so the use of the plural makes sense. I wouldn't use an apostrophe, but if I had to, it would come after the 's'. My reason for this is that other managerial positions don't take apostrophes. I think the relationship is not really one of possession- we don't say 'Accounts' Manager', so I would say the 'Americas Manager', or, to avoid any confusion, use a term like Regional Manager and then classify it. Others may well see it differently.
With the direction the thread has taken, may I ask for thing to be a bit less heated? Posting on forums can lead to misunderstandings- forums are asynchronous and we are posting across continents to strangers. Disagreement is fine, but when arguments break out, the problem is that they stay there for everyone to see, and they don't greatly benefit the learners trying to work out the answer. It is easy to get the wrong idea from someone's post, and equally easy to make mistakes- I'm the king of typos and other errors. However, in-fighting ultimately just sours things, so can we please try to avoid it? If someone posts something you feel is an attack, it is a good idea to use private messaging to ask them about it, and ask them, if necessary, to remove or edit the post. It is better not to deal with these things in public, because long after the argument has subsided, the row remains onsite. Undermining one undermines all in the end, and no one benefits.