Probably a virus, not a lot you can do about it, just cut the ugly leaves off. Kentias are pretty prone to viruses.

Organic matter does break down quickly and is likely to leave a heavier soggy medium. You'd probably need to repot more frequently if you used bark (or other wholly organic medium). You'd be better off sticking with your sand.

Hi! I am the opposite of an expert, we just got our Kentia (Manny) a few weeks ago. So I really hope some of the experts jump in! But I love Manny too and have been reading around trying to learn how to take care of him, so I figured I might as well get the conversation started. I've read that brown tips can be caused by dry air. Manny has those a little bit, and I've been misting him daily - not sure if that makes a big difference or not. I also read that yellow and brown spots can be due to overwatering, (though I'm sure other things can cause that too.) I don't know if what you're describing is overwatering, I think it's more than I've watered Manny (only watered him once), but we've had him for such a short amount of time I probably wouldn't know if I was under watering yet. http://www.ourhouseplants.com/plants/howea-palms http://plantcaretoday.com/caring-beautiful-kentia-palm-howea-forsteriana.html I hope some of the experts give feedback since they can give you answers based on experience and not just stuff they read!

It does look like an over watering problem. But also, Kentia Palms are single trunked plants so you have a number of them crowded into that pot. Most likely they were originally all planted together as seed and and the weaker ones are dying off. When water drains out of the pot do you remove it or does the bottom of the pot sit in water? I can't see any other reason why you'd get those marks on the leaves.