The second SCM gives you redundant SAS connection. But MPIO driver needs to be installed on your OS. See Special instructions for installing dual Storage Control Modules for details.
With the second ESM, you can connect your compute modules to two seperate physical networks, or you may configure SFT (Switch Fault Tolerance) teaming for redundancy.
Yes, and this can be a much cheaper way to go if you want all the modules, just beware there is also now a MFSYS25V2 which uses a new backplane and management module, and different version of the firmware than the older MFSYS25.
Btw please take note that one Dual Gigabit Ethernet I/O Expansion Mezzanine Card AXXGBIOMEZ is required for each compute module that needs to access the second switch. See FAQs about the Intel® Gigabit Ethernet Switch AXXSW1GB for details.