I'm a newbie to UE4 and I am having trouble with the level blueprint, gate, flipflop nodes

You can play a cinematic from a blueprint. I created an elevator that goes up and down based on a line trace. The elevator movement is a cinematic. I did not use a lerp tranform. here check it out.

I would create a single BP for the secret doors and set all the functionality of the secret door in that BP. Drag and drop them in the level for every door.