There could be a ammonia spike of sorts which is not agreeing with the shrimp from the decaying plants, throughout the day while the plants are taking in nutrients (and maybe some excess ammonia?) it may be very low but once the lights go out and the build up of what the plants aren't using occurs, this could be the problems with the shrimp. Considering your saying there dieing at night?... this sounds very feasible.
Not to sure on the plant problems, it could just be lack of ferts and carbon as you say.... TPN whether it be brought or made is actually a weak solution and even dosing 1ml per 20l or so will still be quite weak. Whats the flow like round the tank?... i know HOB filters aren't very good at distributing water evenly due to the waterfall effect.
Ive never had staurogyne melt so couldn't really comment on what Ian has mentioned, mine has always been submersed grown before i have purchased and it just continued to grow everytime after planting, maybe yours is just melt, as maybe the HC?.