I believe the last 20% is gained purely from Makinas House.
The actual bad ending only happens if you actively choose it in your room, when everyone is at 50 lewdness or above.
As for the alternative one, i just considered it alternative since i tend to go for pure runs, but it is more so an extra scene that happens after "The end" screen when you go to the capital, if your "Lewdness" is above 50.
You might be able to go for all the scenes without hitting that threshold, but considering you want to see everything, that may not be a problem.