Senpai/kohai relationships

As everyone knows, the relationships between senpais and koihais are very important in Japan. What I would be interested in are some more complicated constellations, such as:

Let’s take the Sakamichi Series. There is a (group) hierarchy here: 1st Nogizaka, 2nd Sakurazaka, 3rd Hinatazaka. So, for example, what would the relationship (and addressing -san/-chan) be between a first generation member of Hinatazaka (last link in the Sakamichi chain) to a fourth generation member of Nogizaka (first link). Who is Senpai, who is Kohai?

A second example – in the same group (e.g. Hinatazaka) there is a 1st gen member who took a 2 year sabbatical, is now 21 years old and has spent a total of 3 active years in the group. Then there is a member of the 2nd gen who is 23 years old and has spent 4 active years. Who is the Senpai?

I am also interested in whether these rules are clearly defined in Japan, so to speak, or whether there is sometimes a lack of clarity there too!?

https://www.reddit.com/r/japan/comments/ytp8z4/senpaikohai_relationships/