I have only two suggestions. I see that you did attempt what I suggested, I will suggest it again, in a slightly different way. (Not that you didn't try, merely saying that if we were in the same room, it is the first thing I would try! )

Slide stop still in place, magazine out, pistol functional--

Draw slide all the way back and with a thin rubber grabbing assistance (think rubber kitchen jar opener, a short bit of bicycle tire innertube, ANYTHING that enhances your grip but certainly not a pair of pliers) try again to get a hold of the guide rod from the muzzle end and slowly rotate it a few degrees or slowly and carefully, a full 360 degrees, plus or minus.

Basically, anything so that the guide rod ends up somewhere other than it's orientation for the last 20 years.

Then again attempt to remove the slide stop and move the slide forward slowly off the frame. Must know: the hammer absolutely has to be cocked fully (not half cock) if you want to draw the slide off the frame on a 39-2. This is because the sear release lever will not allow the slide to go forward unless it is able to fall. At hammer half-cock, that sear release lever will not go down in to the frame and allow the slide to pass over it.

If the sear release lever cannot lower in to the frame, the slide isn't coming off the frame.

Edit to add!
I just did this repeatedly on my 39-2 and I am going to say it is quite possible that all you are missing here is that you need to start with the hammer fully cocked. Once the slide moves forward enough, it will drop the hammer for you -- be ready to catch it.