Perhaps someone has the canonical answer, but when I travel, I don't care much about these things; So, this is not so much a direct answer to your question, but a suggestion of an approach.
You have proof of purchasing this ticket. Just in case, carry a copy of that with you.
Emirates is responsible for getting you from A to B, though I suspect there's some code-sharing going on, as, as far as I'm aware, Emirates flies neither from Brazil to Panama, nor from Panama to the Bahamas.
It's quite possible you will get all your boarding passes in Colombo, but it's probably more likely that you will have to pick up your boarding passes at some of your stopovers. I'd guess Brazil and Panama, but, who knows. When you check in, they'll tell you, or you can ask, and, if you want to be certain, confirm and ask again.
You don't mention where in Brazil is your stopover. Assuming it's GRU (Sao Paulo), it's most likely your luggage will be checked through from Colombo to Nassau. When checking in in Colombo, they'll tell you. If you do have to pick up your luggage somewhere along the way, you will need to be allowed to enter the country in question.