This just has short after stories and is standalone:
https://vndb.org/v29724
Yes it does have a plot and a good one at that but in the end that is subjective, you need to read all 4 episodes to understand the story.
Yes. Essentially this isn't the "series" - it's single VN split into 4 (or actually 5) parts. Each of the four episodes contains single route.
This is emphasised in the Complete Edition, where to unlock each route, you need to complete the previous one, and then after stories get unlocked after completing all four main episodes.