Sorry for the delay in getting back to you, to answer your earlier questions:
- F12 is incompatible with RPG Maker software for the most part which is why you were getting that error.
- The OG FF IV did have 5 party members, however in Alter Destiny, you only get 4 for story reasons.
- Yes, once you make a save past the 7th area (after jumping across the platforms) of the Lunar post-game area you won't be able to go back. It's advised to make multiple saves before you get to that point.
- The first weapon in the Lunar Subterranean can be found virtually the same way in the OG, by following a particular path.
Congrats on beating the post-game! :D I hope you enjoyed it.