Re: puzzling, persistent substrate dampness
If there is no free water in the bottom of your tank then there is no real wicking going on. I feel like this is likely not your issue.
Plants might die from lots of other factors, insufficient light, not enough nutrients or the wrong kind of nutrients or too much nutrients, or they were just plain unhealthy to begin with etc. What makes you think the substrate is too damp? Your more recent picture shows a number of plants in the substrate, are those plants doing ok now or are there problems with their growth?