Can jailbreak be retried with opt-restore in iTunes

Trying to jailbreak with SP 0.8.5 with the new 5.0.1 untether, the first attempt failed with a -1602 error.  I learned I had to have ONLY the USB cable connected, not the power too. ok fine. But on multiple subsequent attempts it failed in other ways, such as stuck on “preparing to install”.  On all of these subsequent attempts I was just using iTunes with opt-restore and choosing the SP firmware in Application Support.

So then I just re-ran SP, this time with only the USB cable connected at it worked.

The question is, is it actually possible to use the already created IPSW to restore in iTunes?  Or must SP actually inject those .dfu files for it to work?

If the latter, then why is there no option in SP to bypass the IPSW creation entirely and just skip straight to that .dfu injection, assuming the appropriate IPSW has already been created? And furthermore, some obvious documented indication of this would help.