This is a timeline where Gaster allowed Emily to attack the Capital. She wanted to have Asgore yield and settle this without bloodshed.
...
Sadly, Asgore struck himself with his weapon. Jeremy and Undyne ran in as Emily was stumbling backwards. The three, horrified, just stood together and realised how cowardly Asgore was. Not able to accept defeat and would rather die than to face it.
It would've been reasonable for Jeremy to become King since he was the closest to Asgore at the time, but he decided Emily deserved this title. After all, he put her through so much and didn't deserve some kind of reward for pain and suffering.
Hybrids and Monsters live in harmony underground and the hybrids even showed the monsters how to get to the surface without breaking the barrier. Zoey and Undyne are both co leaders of the guard as Jeremy kept his title.
Gaster passed on, illness wise, as Emily drowned herself with everything she could've done to stop Asgore. Of course, no monster blamed her. ... but she had that feeling she could've done SOMETHING.
You may ask her questions.
(Pls give her love, cuddles, maybe some tickles, and cookies!)