I removed my seats a couple of years ago to do some upholstery repair, so I am going to be working from memory on this (my car is in a storage facility for the winter so I can't get at it to check).
From my memory, I will state what I think has become obvious and that is if the seat can't be moved forward, a regular human is not going to get access to the rear mounting bolts. Now for some possible options!
In the photo provided by Midnight_Raven, you can see the cable which connects the right and left drive mechanisms on the seat frame. If you remove the cable and find some kind of square tool (same dimensions as the drive cable) that you can insert into the cable receiver on either side of the seat, you can turn that tool by hand which should cause the seat to move backward or forward (you will need to figure out the direction). As you turn the tool, the seat will skew in its tracks and jam, so you will have to do a few turns on one side and then do a similar number of turns on the other side. It will be slow and painful - my back hurts just thinking about it! Of course, this will only work if the drive mechanism is not buggered. You mentioned rust. If the worm gear drive mechanism that the cable fits into is seized up, then this is not going to work. As a note, you mentioned that you tried turning the 'rod' ( I assume the worm gear) by hand. This generally won't work. Worm gear mechanisms tend to be a one way type device. They jam if you attempt to apply torque on the output (which is what the long worm gear is on each side of the seat). I will let the mechanical engineers explain why!
If the preceding doesn't work, one last hale-Mary. The seat bottom cushion / upholstery covers an aluminum pan which forms the base of the seat. I don't know how that pan is attached to the slider mechanism as I just removed my center cushion, not the side bolsters. If you can pull up the center cushion from the front (you won't be able to remove it completely as you need access to the back of the seat to do that) and peel back the side bolsters and the bolster foam, you may be able find the attachment points for the seat pan. If it is screwed onto the frame, you may be able to pull off the seat pan which might give you access to those two bolts at the back of the seat. That is a lot of mays and mights. Somebody who has pulled the bolster foam off may be able to advise as to whether I am out-to-lunch with this suggestion.
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Further thought! If you remove one end of the cable connecting the two sides of the seat, you should be able to insert the inner drive cable into the chuck of an electric drill and then use the drill (on the slowest speed) to advance the seat mechanism. You will have to switch the cable from side to side in order to advance both sides of the seat mechanism.