Can quest items be taken and left at the OLs keep. I assume that they can be but it also seems quite strong since the Heroes can never get to them and I have 1-4(5 w/beastman lord avatar) Lts. to go and pick them up and take them back.

I am currently doing the plot with the scrolls and you are trying to cover up the sun. I am using the Demon Prince Avatar. Each quest item can be placed in 1 of 3 spaces and 2 out of the 4 items I can place just a couple roads from my keep which makes those two really easy. Since I know where they are going to be placed I can just send out my Lts to pick them up and wait to place them until I get there. If I send my Lts that have Eldritch minions the Heroes have very little chance of surviving an encounter so even the items place a bit further aren't hard to bring back. This doesn't give them much recourse to stop me from successfully executing my plot and winning the game.

Is there anything I might be missing or is this plot just really hard to deal with for the heroes?

Well if the Heroes let the Beastman Lord carry all the scrolls home, and then let him carry the Sunstone to the Plains of the ruby Gate half across the map, they deserve to lose, methinks. So many opposrtunities to stop you, and all it takes is for you to lose one of the items and the victory is not yours.