I see, but a problem exists. It is that the swap disc can't use TOC.
Of course But it's not a problem.
Solution 1: User inserts disc normally, DIC reads the TOC and leadout (to know the image length), then ejects it and asks to do the swapping (like, "insert the trap audio disc and press any key" => "swap the disc and press any key").
Solution 2: User runs DIC after swapping, DIC reads data and subchannel into .scr + .sub, then ejects the tray, then inserts the tray, then reads the TOC and leadout, then cuts the image to tracks and does the descrambling. Or you can ask the leadout value via .exe parameter (to know how many sectors to read into image) and read only TOC after ejecting/inserting the tray.