My server has merged, I have experience how server merge work. Now I post thing here as your lifesaver (if your server is merged next) because no one told me what happened and what to do, everything I tried to find and found in this forum just someone's problem with a 'very difficult to understand what happened' manner.
What happened when you login after server merge ? It's exactly same as when you create new account, it shows you a map and ready-select you in 1 kingdom (called recommended kingdom - where your 'used to be your house' already in there). You can choose another kingdom or just ok with that choice.
This is your lifesaver: you have about 5 - 10 minutes right after server merge end to do the very correct choice. For example server merge end at 5:00 pm, you have from 5:00 pm to 5:10 pm to select any kingdoms you want to join, during this time there is no strongest kingdom yet. I chose one account in Skarn (strongest kingdom) and another account in Athaly (which I thought the strongest kingdom since they ruled half of merged servers), all succeed.
After that time you can't choose strongest kingdom anymore, because it already define the strongest kingdom.
1) If you are not familiar with that login screen, simply create new account to test, it's exactly the same.
2) It's very hard to migrate to strongest kingdom since lilith changed condition, power index, it relates all players in kingdom and can't reduce it to the amount you want. You should choose the correct one or struggle with your choice.