You kinda are misinformed. Most of your facts are not wrong per say, but inaccurate.
1) People aren't contagious during the incubation stage (2-21 days). However it can be hard to diagnose the first symptoms so yeah someone infected with ebola could go to a baseball game with a little fever but even there he is not highly contagious and for other people to be exposed, they need to be in contact with either cutaneous lesions, or body fluids.
2) There's absolutely no evidence (and a boatload of research have been done about it) that any pathogens (bacteria, virus, fungi, ...) develop resistance to treatment because they are exposed to the said treatment. Pathogens mutes naturally and sometimes develop a resistance to the treatment. The treatment did not produce the mutation, but will select the mutant strain resistant to treatment.
3) There's also no evidence to the fact that any virus could change its transmission pattern. To the scientists knowledge no virus ever did such things.
The outbreak in West Africa has been traced to a 2 year old that lived in a small city/town near the Guinea/Liberian border. From that one patient in a small town in Africa, it has resulted in a few thousand cases. So let's say that boy instead lived in NYC, I am not suggesting that people would get infected at the same rate as they would in Africa where conditions are perhaps less sanitary and resources more scarce. I am suggesting that the sheer size of the population of NYC and the fact that population is more mobile means that even if they do get infected at lower rates, it could still result in a larger potential for outbreak because once again the spread of a disease is not solely a factor of it's transmissibility. It's also depends on factors that exist in the exposed population as for example the U.S. may be more sanitary but a city like NYC also has a shit ton more IV drug users or people living on the streets which means if they got Ebola, their chances of spreading it are a lot higher than say a person that lives a more sanitary lifestyle.
Another factor is the advanced warning. Part of the reason outbreaks like this get contained is because the fact it broke out in a small city in West Africa which means that by the time it ever gets to a big city, that big city is usually already aware of the potential for outbreak. Case in point, the US had significant advanced warning of the risk for Ebola due to the outbreak in West Africa. I know people that got a fever and who ran to the hospital to be sure it wasn't something worse because they had heard about the Ebola threat. That is a far different scenario than if an outbreak originates in a large city with no advanced warning. Those same people probably wait out the fever longer.
I wasn't suggesting that pathogens develop resistance because they are exposed to treatment. I was suggesting which is what you are saying that in being exposed to these treatments natural selection would occur and the strain that is resistant or is airborne would become the dominant strain as the ones that aren't resistant or airborne die out. The point being we may not have knowledge of those strains right now because they are only a small part of the population.
So I am not saying Ebola can change its transmission mode in the sense that a strain that was not airborne can suddenly become airborne. I am saying a strain of the virus that is airborne may already exist but we just haven't encountered it because the strain that is not airborne exists in far greater abundance right now. The first 2 Ebolaviruses were discovered in 1976 then another one in 1989 then another one in 1994 and the final known strain in 2007. That is 5 strains whose discovery is separated by over 30 years. So we can't say with any degree of certainty that we now have found all existing strains out there because the reality is we typically don't find new strains until there is an outbreak. This is on top of any sort of human attempts to weaponize the disease.