You need to rework the whole save game format, turning it from a plain text file into a unreadable binary. Otherwise the password protection is pointless as you can easily read the savegame or change it in a way you can actually load it without any trouble.
This will also affect the modability of the game, as well as debugging. For a plain text file you don't need any further tools, for a binary you need to add a kind of compiler which can transform the whole information into a readable form - of course only available for the developers, not for the end users.
You wouldn't need to make it a binary. You could encrypt the file over and over using each of the player passwords as the keys, but then you wouldn't be able to access the game unless you had all of the players keys (ie all of the players are present.)