
Somebody please explain it to be what os going on with Erin and the ml.! What is her connection with all of them...

I'm not sure who's the ML you're referring to... but here goes. Irene (mc) holds the soul of the goddess of balance (Erin). So, she is not necessarily her reincarnation, but Irene is more of a vessel. The demon god and heavenly god worked together to reincarnate A PIECE of Erin's soul (now Irene) back into their world, so that she would used to resurrect their lost sibling --- Erin.
Ren, the pixie, and the dragons from the first chapters were all followers/close companions of Erin when she was first revived as a human mage of the middle land. This was BEFORE the demon god and heavenly god reincarnated her for a SECOND time in their world as Irene (mc).
So the demon lord and heavenly lord also orchestrated the encounter behind Karaban and Sein (Irene/MC's parents) so that they will give birth to a baby with both dark and divine powers.
((Sorry for the long comment I hope I helped explain it... if you have confusions or questions I will try my best to answer ))

Somebody explain to me why they reacted like that to the doctor....

tbh i think they had sex...on the author pixiv you can see this https://www.pixiv.net/en/artworks/76545901
someone posted it here a few comments back

It's official on Cmoa (a official Japanese site that has the 2 volumes & single chapters versions) includes it on their site, so kou & ginga official had sex before the main story
https://www.cmoa.jp/title/152580/ (there listed on the 2nd page) on the volume version it says vol.2 has it but can only be read digital so I guess the actual physical copies don't have it
I have so many issues with this story especially mistreating women and the cringy moments but I have to read how it will end