Hi Zhongwei,
While liveCache is related to MaxDB (it can also run on HANA), you should seek help with this content in the following SCN space:
For your questions, I don't know what SPP stands for, but:
1) Yes, liveCache is a database and no, Oracle isn't useless in this scenario as the APO/SCM database runs on it. You need both databases (it can also run on HANA, MaxDB, SQL Server, DB2, ASE).
2) Can't answer as I don't know what is SPP in the MaxDB context
3) Yes, they're synchronized and if you have any inconsistency you might face errors during simulations, etc.
Regards,
Luis Darui