To clarify a little further on point #1:
You cannot use _id as the date field in the Date criteria. Technically, you can incorporate the workaround of using the $expr in custom criteria of Online Archive that @Hartek_Sabharwal mentioned above.
However, the custom query will likely not be using an index and the archival process itself will be likely slow. We have mentioned below in our documentation :
For custom criteria that use an expression, Atlas might first convert a value before it evaluates it against the query.
The recommendation is that you create a new indexed date field and use the Date criteria to archive. This is the right approach that will optimize/improve the archiving speed.
We wouldn’t recommend custom criteria and using $expr and using _id due to the slowness mentioned above.