Wow. I solved it perfectly using both of your advice.
Saving algorithm of my game is like, picking up a slot at the very beginning of the game and overwrite that slot every time after major events happen. (including entering the puzzle zone.)
Since Autosave and Autoload function of your Check point plugin only use Autosave slot(Slot Id=0), I made some changes to the code you made.
I changed assigned Id for your ExecuteAutosavePlus and ExecuteAutoload function, which is 0 as default, into '$gameSystem.savefileId()' which you mentioned.
And now, whenever I run your Autosave/load function, Not only it works beautifully, but also keyboard assigned for autoload function too works well.
I wrote the above information in detail to thank you and maybe to help you update your plugin. Maybe you can add the function of Saving in/Loading from the last save file, instead of autosave slot, while you update the Checkpoint plugin. I'm sure there will be someone like me who will find it useful.
Again thank you very much for your kind and dedicated support.
