Thank you! Now I see how it works.
Sorry, if I ask something obvious not related with the mod: I am new in BG, didn't play it before at all.
As far as I understand it's possible to select quick spell by right-click on quick spell icon, but it doesn't work for this multi-class sorcerer: nothing does by right-clicking.
Also when I created flat sorcerer to test, it is not able to cast spells: learning spells works as you described and as it works for fighter/sorcerer and spells are visible in magic book, but icon to cast spell is inactive as against fighter/sorcerer.