Dear Ashu,
1) I think you should not worry about whether users are posting or not. Assessment cycles are run as part of the month end closure activities. And pls note that COPA is time stamp based so is your delta also. So no conflict between your delta run and assessment running at the same time.
Now about no of records, i dont think assessment brings so much of records unless you have pretty huge organisation.
2) There is no duplication of data. Assessment cycle posts using secondary cost elements into the defined cost centers and there is no chance of data getting duplicated during the delta process.
Regards
Gajesh