1) E2800 does support an SSD read cache - it is only for read operations.
2) I'm not sure what is meant by high performance tier - E-Series does not do auto-tiering.
3) You do not need to buy any additional licenses for E-Series - all feaures are included.
4) Yes it is absolutely possible. You will need to be sure to purchase the controllers with iSCSI "optical" base ports. You cannot use an FC HIC with controllers using RJ45 iSCSI base ports.
5) Yes you can convert the optical iSCSI to FC and vice versa. You will need a feature code. Note, this is not a license, and there is not additional cost involved.
6) You cannot mix SAS and SSD drives in the same Volume Group or Dynamic Disk Pool. You can have multiple volume groups on the same array with the same type of drive.
7) The CPU cores are not assigned to a specific LUN.