US Immigration from Thailand

Hello

i hope this is the right place to post this question / questions. if not please feel free to move the thread.

Our situation:

I'm the petitioner for my fianc who is currently living in Thailand, im in South Florida. We have filled our K1 an received our NOA1 in May of 2022 so we are currently in line like everybody else.

My fianc has 3 kids one 17 who will be staying in Thailand because next year she will be attending collage one who is 11 who will be leaving with her mom (My Fiance) when the K1 interview process is over and she an her mom are allowed to enter the U.S. Also one who is 9 who will be staying with his grandfather for about a year after my fiance an step daughter leave Thailand an have gotten settled here in the US.

All 3 kids are listed on the I129F in the appropriate place on the application. What im wondering about is when our I129F / K1 finally gets to that interview stage. It`s my understanding that the child will have to do a D160 along with her mom, get medical records, immunization records, police back ground check like her mom an then attend the interview with her mom. is that all or am i missing something?

For her son who is 9 who is going to stay with his grandfather for about a year will he need the same as his sister when we get ready to send for him.? or will he need completely different stuff?

I read somewhere that we might be able to get his grandfather an my fiance`s son here on a B2 visa an then maybe file for a change of status for my stepson.. which doesn't make any sence to me. But then again i think im in a state of confusion most of the time trying to figure all this stuff out.