Desoldering CPU from PCB or holding it in reset
This depends on the design. To dump contents of the flash chip it must be powered, and the driving device (driving outputs) on the PCB must be in high impedance (Hi-Z) state. If you have flash connected to the processor directly and you are sure, in reset condition, having consulted its datasheet, that it will pull all flash-connected/related signals to Hi-Z, then you will be able to dump the flash easily. However if there's any active buffer device in between which is not going to Hi-Z on reset condition you will not be able to perform the dump.