Why is Flame Finger an Ob 3 spell? It seems less complicated/powerful than shards (Ob 2) and more on the level of Magic Whistle (Ob 1).
Also, are Mend and Minor Maker really sustained? Are they designed to be temporary fixes that are used until someone can use normal skills to permanently fix things?
While I don’t know about the first question (though I think it has to do with the Fire facet), I do know that the source of Mend and Minor Maker spells comes from A Wizard of Earthsea. Ged binds driftwood together with his magic to make a boat. It falls apart as soon as he release the spell.
Without the book in front of me, I would guess that Shards requires a material component (thereby reducing the difficulty) and the Flame Finger is a manifestation from nothing.
Try making both spells in the Magic Burner, earth is the easiest element to control (lower base Ob). Fire is 1 higher.