Well, all conflicts should be resolved locally. Without a software capable to do so it probably will be impossible to upload changes in some cases.
That is a possibility, I am not sure how conflicts are handled. In some cases, like when you want to up a better image or improve texts but don't know how to use git software, this is not a bad bet.
Actually, based on the text I feel like you can only ADD a file, not change something in an EXISTING one.
You always could change something in an existing file as long as it was open text, ever since we've started with Git. I've did a quick test using an image, it looks like it will always attempt to overwrite the existing file in the repo when you use the new upload system. I am not quite sure what happens in case of conflicts though...
But even then, you can just branch out in such cases, branches shouldn't be too hard to merge together and deleting them is very easy.