Usability of scrolls isn´t level/chapter restricted.
I´ve always used them as a mercenary, summon demon on stone dragon, shrink on the ice one (a fitting retort to his pompous speech), fire rain to dispatch the orcs guarding the Valley entrance for an early experience boost, etc.
It´s one thing which makes a mage class comparatively weaker than the other two.
I´m not sure if I remember this correctly, but to use a spell scroll you need to click on it in inventory to assign it a key first, say "5", leave inventory, then press "5" to make it active (shimmering effect occurs) and to finally cast it you have to either hold left mouse button or ctrl + up arrow.
Edit: Just realized casting from runes works the same, so this was most likely redundant to mention. But casting from scrolls really should work regardless level/chapter.
And yes, some hunting skills such as extract heart are good to have as a mage, though it´s possible some of the ingredients can be bought in later chapters, at least fire lizard tongues (for fire rain) can, iirc. I´m not sure about hearts (summon golem, summon demon) or troll tusks (shrink).