how to create a staging table

1. Staging table is just like any other table but the difference is that we do not treat it as the main table.

2, 3. You can write the data to the staging table based on your use-case. If it has a primary key column, then you can write it via Write to DSE but if it doesn't have a PK (importing excel to DB) then you can use the smart service. And based on the same question, you can decide which way to clear and populate the main table. Either use delete from DSE smart service, or run a SQL query using a Smart service or create an SP. 

This does not require much Appian expertise as all is done in the database.

Custom Data Type (CDT) Design Guidance - Appian 22.3