S3 is immediately consistent for new objects unless the service received a GET on the object before it was created. It's easy to use this to make an immediately consistent system.
Yes. HTTP POST (new objects) is immediate, HTTP PUT (updates to objects) is eventually consistent.
If you want to use this to create new objects all the time, rather than update ones you already have, you now have to keep track of which objects in your bucket are "old" and should no longer be there. But yes, totally doable.
