It takes a while before non-beta release of 4.2.x series is out. So I wanted to make the bug fixes available to those who do not want to (or cannot) use beta releases.

I suppose 4.1.9 should not be considered as an upgrade from 4.2.1, right?
Therefore, only 4.1.8 should update to 4.1.9?

Depends. If you have upgraded to 4.2.1 only to get fix to some of the issues (not to get new functionality), then it might be better to downgrade to 4.1.9. It might be more stable than 4.2.1.

Just curious...

Why have you "back-propagated" a lot of bug fixes into 4.1.9 instead of releasing 4.2.2?

I suppose 4.1.9 should not be considered as an upgrade from 4.2.1, right?
Therefore, only 4.1.8 should update to 4.1.9?