Is there a way to up the interval that Active Scan uses?
No, it will scan machines as soon as they are seen by a domain controller, or all machines seen in the last 14 days if you restart the service
Is there a way to do a one-time asset import from Active Directory?
yes, you can use scheduled scanning with "scan now" to do an import.
Is there a way to do a one-time asset import from CSV?
This will be included in version 5.1 (in development)
What is the best way to avoid duplicate computer assets if using multiple scan methods?
Duplicate's shouldn't occur, can you please send screenshots to support@lansweeper.com