Re: Access to rail screws/nuts
From the looks of the pads, there must be snaps, velcro, or double-stick tape behind them to hold them to the sides of the cockpit. I'd try and stick a narrow, dull, blade behind them and pry gently at the corners, while looking closely at what shows up behind them. Assuming they come out fairly easily,
you may or may not have access holes to the back of the deck where the rails are mounted. If there are no holes, you'll have to make some to get access.
edit: I looked a little closer at the first two pics, and it looks like something is sticking down below the backrest cushions. What is that? Also, do the seat cushions lift out? If so, take them out first, then do the pry gently thing at the bottom of the backrests- try both bottom and top corners to see what is revealed.