I didn't read all of this but dont you somewhere have to modify the fuse bits to go to/from internal/external oscillator? Also I read somewhere that the internal osc may work with the serial port but bc of its inaccuracy sometimes needs calibration. I prefer to add xtal, caps and a 5 pin header so I can re-program my sketch in-circuit. The greatest savings is in not having to include the serial chip in the design. I have a dedicated chipless old Uno that acts as a programmer, one Avr Dragon for the fuse bits and then some home-made shield for burning the bootloader.