0.5.13a windows 10
If you buy spells and reload a previous save, you keep them.
I did the first mage quests, bought a few spells but reloaded the "start of day" autosave due to a mistake I made. I still had all the spells I bought previously unlocked and can use them even if they are not available for buying yet since I have to redo the mage quests (tested with invigorate).
I tested the same thing but reloading a previous manual save instead of the autosave, same result.
Steps : (in the same day), do first mage quest, buy spells, save, reload previous save.