I've been thinking about the Menu bugs and how to save you some work while also, Improving the user experience. I worked at Microsoft for some time and my time there showed me the value of consistent patterns in an interface.
Using the same menu path that the stock "World Type: superflat" uses will accomplish this by making it so you don't have to write different code for each version of Minecraft and it will offer the user an experience that is similar to the one they likely already know how to use.
Here's how the flow would work beginning with the standard "Create New World".
Now users click the "More World Options" button;
Then users click the "World Type: Default" button until it says "World Type: Terrain Control". This is pretty standard so far. If users have ever customized their Vanilla world or used mods like Terrain Control and Biomes O' Plenty in the past, this will be familiar to them;
Here's where your changes come in. There's now a "Customize" button under "WorldType: Terrain Control" just like the "World Type: superflat" has;
This leads to your menu which used a standard single entry point and exits via the "Done" button to return to where it entered. Nice and clean! No having to code for options already handled by the previous Vanilla menus and no having to code for the different menu paths of the various Minecraft versions.
Exiting your menu brings users back to where they entered. From here, users can hit the "Create World" button (which creates their world) or the "Done" button to go back to the previous menu that has the "Allow Cheats:ON/OFF" button and other options they may want to review or change.
Less coding headaches for you and a better user experience.
I would love to hear feedback from the rest of your trusted councilors on this thread as well as yourself.