Answer to your Problem 1:
It seems you are using Promises in the change event, but unfortunately, you can't use it in the change event.
So it would be best to stop using the synchronous process for GET API in the change events. And use kintone.app.record.get()
and kintone.app.record.set() within the GET process that was executed asynchronously.
Answer to your Problem 2:
You need to set  in the value of a table.
I hope this answers your questions.