You're correct, you can access the 128MBit SPI Flash as any other read-only memory mapped memory -- you can execute out of it or load data (you can also write to it but need to use a seperate channel, it's not directly memory mapped to write).

You're also correct on the sizes of the ICache and Scratchpad. You can execute code which resides in the scratchpad, but can't store data in the I-Cache.