All the Metal Maid options after the first assume that you have Pure Crafting, but the first one can grant you proficiencies instead.
Intended, basically you can grab the proficiencies and then be able to craft alloy simple and exotic light weapons at half cost, but you wouldn't be able to craft any other pure metal items.
Is Lunar Dial going to get a forbidden school for people who aren't obsessed with knives? 
Sounds like an intesting idea to consider.
Could also make a feat for allowing Luna Dial to work with any weapons.
Also remember that Modern Maid also allows to use Arsenal weapons with Luna Dial.
The only other Touhou maid I can think of is a nuclear powered robot. Who may also have thrown knives at you.
Kudos for you to remember that ending. Reimu managed to break/wear down the poor girl though, I even wrote some fanfiction where she's found and repaired.
Anyway, about the higher levels of metal maid assuming you took the pure crafting, maybe it should be something that can be taken twice?
Besides my previous reply, remember that all the servant options can be picked multiple times by sacrificing higher level picks.
