Welcome. First let me say that you will likely need to replace the seat riser which has the adjuster mechanism in it. As for taking it off, this could be the pain because of the location of the bolts.
The second step which iyou mention is to ....
Move the seat to the full rearward position.
This step cannot be performed because ...the seat will not move rearward (backward)....which prevents getting to any bolts under the front seat.
Also not sure I understand why I have to replace the seat riser....that part works...all other functions of the seat perform correctly except for the seat moving backwards.