As I already said, multiple frames would've been rendered at the same time.2. The article CLAIMS this that each frame took "took multiple hours to render" and that the movie is 240,000 long. This is because the article is FULL OF SHIT!
Again as I already explained, they had access to a much faster low-quality renderer capable of rendering scenes in realtime. The article also explains that interative process like what you described was used during the final rendering process.You simply CAN NOT develop a movie "blind", you have to make something, render, examine, make changes, render, examine, render, etc.
The fully rendered movie would've taken months to completely render, in addition to months of rendering frames that were ultimately thrown away, either to be redone or cut out of the final movie. Avatar took a couple of years to make cost between $200M and $300M to make depending on who you believe. The motion-capture filiming took a month, the live action filming another month. The most of the time and money was spent creating the computer animated parts of the movie. It wasn't and couldn't have been done in a month.
The fact is that PCs are many orders magnitude of rendering movie quality visuals in realtime. Even the massive render farms used to make these movies are several orders of mangitude away from rendering them at full quality in realtime.