Are the SPI pins on the ICSP header of an Arduino Uno "separable" from the correspondent I/O pins - when one chooses to the use the ICSP header can the corresponding I/O pins be used for something else?
Actually, it can. You just can not use them at the same time. One needs to be careful to wire it in ways not to comprise the shared functions. Where MISO can be used as GPIO-INPUT and MOSI/SCK can be used as GPIO-OUTPUT, avoiding any pull up/downs, etc... As such electrical items may interfere with the ICSP.
For example I use the ICSP programming, along with the MISO, MOSI and SCLK pins as hard SPI and use them as Bit Bang one-wire, all on the same board and same wires. I simply ensure the SPI is properly configured each at each use.
One additionally needs to ensure that any peripherals on the shared SPI/ICSP lines are guaranteed to be in reset. Such as a pull down on such devices reset pins. Or a Pull Up on their ChipSelects. Noting that ATmega's GPIO pins are Hi-Z out of reset and during the ICSP.