Can you lower the greenhouse temperature at night, or will that have a negative effect on the other plants you grow in there? Hb-101 is supposed (according to their website) to be used in conjunction with an actual fertilizer, it's not a stand-alone thing. Fuchsias also do not like too much sun although given what else you are growing in there I'm guessing it isn't getting too much.
'Dark Eyes' is a deciduous fuchsia so I also wondered if it was losing its leaves because of the time of year, but having seen the picture that doesn't seem to fit. Are there any buds at the nodes on the dead branches? It doesn't look like it from the picture.
I don't think continuing with the hb-101 serves any purpose, and it probably doesn't need fertililzer at this point. Fuchsias don't like to be kept too dry.
I think your best bet is cuttings.