The reason they are not available is because they are part of a Script. When modifying Scripts in RPG Maker, the changes very rarely translate into old saves. Usually a restart with a new game is required. It is an engine limitation unfortunately.
Good catch on the Fire magic though, not many people notice it. I am aware of it and am going to change it in the next few releases. Definitely by the time the Werewolf Cave expansions role around.
I only know fire magic right now so that is why I noticed. You said that the slimes are only weak to ice magic, but they appear to take significant damage from the magic wisp spell from the copper scepter(you may want to increase its wp cost for doing three hits, as well as making scepters two handed and weaker one handed wands because right now I'm dual wielding two for increased magic damage), so are they weak to pure magic damage as well? The only other spells I could use on them were fire which were weaker in the rain anyway, so I never compared it to any other magic.
You said that the changes in scripts very rarely translate to old saves, but that would mean that they sometimes do. Would it be possible in a future release for the crafting list to be updated without having to start a new game each time. either regularly or with an item that updates the crafting list? I already have 28 hours on the game and don't feel like restarting and having to get my stats back up.
Fire Magic is very weak against Slimes, it only does about 10% damage. They are also Immune to Lighting Magic. The reason it feels like the Magic Wisp does a lot is because they have no resistance to "Pure Magic" damage and have rather low HP pools. I will probably boost their Resilience (magic resist) a bit to compensate for it.
In general, it is probably not worth restarting every update as they usually only add one or two recipes which you might not even have any interest in.
I'll see if the "recipe book" approach would work, maybe if I make the Game Update event relearn the recipe it might work... I'll need to test this.
