Need some help with Backup Settings
1) You can simply set the software to delete versions that are older than 90 days on the retention policy window. That should be enough.
2) I'd suggest to do full backups as frequent as possible, that should keep versioning consistent. Doing full backup once a week is a good practice.
3) That's almost impossible to calculate, especially for file backups. You need to take into account the size of the files, amount of changes for that data set and the frequency of the plan runs.
4) You might want to disable "keep number of versions" parameter if you want to keep more than 3 versions of data on storage side. Time-based parameters look good.
5) It's up to you, but in general I recommend running full backup once a week, like I previously mentioned.
6)
This article should help, the process is automatic, but it will take some time to purge the data.