Fair question.
You essentially have a loaded firearm (even though it is not in battery) that needs to be worked on. Since it is not possible to clear the firearm, you need to render it safe somehow. You have to assume the safety is damaged and not blocking the sear since it has been displaced and is no longer in the proper position. The firing pin will probably not be removable. Chances are that the sear is damaged in some way. If the slide actually goes into battery, the hammer might fall. You need to eliminate the chance of the hammer hitting the firing pin.
So how would you make the firearm safe?
The easiest and most direct method of removing the potential of the hammer striking the firing pin if the firearm were to go into battery would be to remove the stored energy of the spring that drives the hammer. It is quick and easy. One pin. The only reason I said to remove the grips first is to prevent damaging the grips as you drive the mainspring housing pin out.