I'de recommend looking through the following article to get a good understanding about LEM retention, there is no option which lets you pick how many days in total it should store the event logs for. You have to do some calculation based on how many events are coming in per month to your lem and see if your settings are correct to allow for that length of time: Live Data Storage Retention - SolarWinds Worldwide, LLC. Help and Support
Regarding the Backup [ https://support.solarwinds.com/Success_Center/Log_Event_Manager_(LEM)/Configure_Backups_on_your_LEM_Appliance] , you are just outputting logs per month into an external location. If you want them imported, you would have to have another LEM somewhere where you can then re-import the backups to. Import Settings from a SIM or LEM Manager Backup to Another Manager - SolarWinds Worldwide, LLC. Help and Support That's where you may require the supports assistance.
Also found this article perhaps it can help: Import Archived Windows Event Logs to LEM - SolarWinds Worldwide, LLC. Help and Support