I don't have a direct answer to the disk load situation, but I have a couple comments.
First, I know schtruck has spent a lot of time getting the disk change to work - so I'm sorry you're having problems
. It does work, I'm sorry it's not working for you in this situation.
About save states: do not rely on save states! They work 90% of the time, or more. But, sometimes they have problems. Always save your game via in-game save. In fact, I recommend only using the in-game save, and just use save states for the specific times they're needed - right before a decision that you might want to go back on, or a boss fight, etc. Even then, make sure you have your game saved via memory card. Like I said, save states usually work - just don't let the one time it doesn't work ruin your work in the game.
Disk cahnge - most games that have you change disk allow you to save immediately before they have you cahange the disk. If you have problems with the disk change, save your game at that spot when the game lets you. Then restart fpse. Load the next disk. Load your save game - you should be able to continue from there.
Disk cahnge - process: menu, eject. choose next disk. The game should read the new disk. You may need to do menu, quit, restart - but that's more or less the equivalent of pressing 'reset' on the playstation.