Don't worry about the tentacles. I sorted it out. I had an open chestpit inside the chest and each time the tentacles spawned they fell down the pit.
I just took out the chestpit and everything was fine.
I also solved the chestblockage error. I had simply forgotten to remove a chestblockage I had placed in the chest's space earlier. Once I removed it the error disappeared.
I can be such an idiot sometimes.
It does make me ask a question though. What are a chestpit and a chestblockage for, since they don't seem to have any impact on the functioning of the chest at all (that I can see)?
By the way, if you put a closed chestpit (trapdoor) inside a closed chest you end up having two lids; one opened and one closed (though that might be something you already know).