Im not sure if I understand you question correctly but:
1) Yes. You can use extended retention on disk.
2) A cycle is full backup. There is no dependency between full backups (unlike dependency between diff/inc to full, since without the full backup the diff/inc backup are useless...). In case of Daily backup failure, the extended retention rule will select the most recent valid backup for extended retention (as configured in the extended retention rules).
3)I cant suggest retention since that's per-customer requirements. When we come to the customer for the first time one of the main questions is "How long would you like to save backups and on each media (Local Disks, Tapes, Tapes sent to Databank, Cloud, etc)?"
4) Same as mention, Secondary copies are suggest per client needs for number of copies of backup. some of my customers have 2 copies (local + weekly to tape) and some have 2 synchronized Disk libraries and 2 seperated tape copies...