If that were true, then the two versions would release more or less simultaneously. They don't. Moreover, having the computer render a new image, even one that's already been framed and properly set up, still takes time and processing power. Which is why for example, minor variations of scenes (such as a LI being pregnant or not pregnant) are typically forgone or pushed to the end of a story to cut down on the workload.
1. if it takes more than that, then dev is doing something wrong
2. you are correct about rendering time. But that is the computer working rather than a human working.
3. LI's being pregnant or not is different, because you don't just need to render the images, you also need to track variables and determine which image to display when.
Now, if the two different MCs were packages together into the same game with different file names and the dev had to write code on each scene to determine which to show, then it would be some extra work.
But he is actually releasing them as seperate packages. so presumably it uses the same file names. So it really is just re-rendering.
The fact dev is doing it as seperate releases is showing a level of competence and understanding. which suggests to me he is doing things right.
My guess is that the reason for the separate releases is because of the time it takes to re-render with the different skin.