During the Elder Knowledge quest at the part where the scroll is obtained, i forgot to pick up the lexion thingy for the quest mentioned in the title. Is there a quick way to go back or will i have to travel all the way through blackreach again?

There shouldve been a lever that you pulled acting as an elevator of sorts. I can't give you direct directions, but yes it's possible to go in from the back. This is operating under the assumption that the lever that controls the gate hasn't/doesn't reset. Best of luck!