1)No, you don't need it.
2)That is one (multi-level cell) of two NAND memory, other one is SLC (single level cell). MLC is cheaper the SLC, also slower too.
3)In the BIOS change the AHCI mode, and boot order DVD/CD first, then SSD/HDD. Saving and exit.
4) It mean organizing the smaller files into a single directory together, as they are often accessed in sequence. That means they write and rewrite the files in the SSD, if you do a lot defragging, you don't want to write data on the SSD too much because that will wear out the SSD.