That is my understanding yes - you decide to cast the spell, that uses that character's turn and the spell is used up. Then you roll to determine if it hits or not
My understanding as well. Spells are a resource that get used up.
Is that confirmed? Also in my first game I was playing that the spells does not get used if fails (cfr. E.g. Dungeon storming rules)
It's my understanding as well. The base rules don't appear to specify. However, in the Wizard class description it mentions that a certain number of spells are chosen before entering the dungeon, and that you can have multiple copies of the same spell if you choose to.
That to me at least hints that spells get used up. Otherwise I see no reason to stack them.
Actually tour reasonment does not apply - of course they get used (and so you are stacking them at the beginning) - we are asking here if the spells get used also when they fail. Basically if you use them up at the moment you decide to cast the spell, and not after seeing a successful resolution.