Amazon import - want to make sure i get this right

Sorry if a double post - i thought i posted but now cant find it.

Currently, I have my Amazon charges in a Category called Pers - Amazon in a group called Personal Spending.

I want to run the importer for a whole year (2021) of amazon history i downloaded.

Q1 - what does the importer do - does it overrite the existing row entirely? or does it overwrite particular columns only?
Q2 - since i have Amazon charges already categorized, will that cause a problem for the importer? or result in dups?
Q3 - I dont fully understand the settings - what does offset categorization mean?
Q3 - what does filter transactions after export mean and why do i need to or want to do that?
Q4 - What is a good way to test if all is working well? I was thinking to find some amazon transactions before running the importer and color coding them yellow - but then what would i look for once the import is done?

Thanks for bearing with me through all the questions!