edward, I understand you're trying to create the most natural media for your northiana, but these are my opinions on your complex medium:
1. I highly doubt that sphagnum moss would grow on the vertical limestone cliffs and crevices that northiana naturally inhabits. I think that most of the moisture comes from decomposing leaf litter and heavy dew. IMHO, a lot of these cliff-dwelling plants can take less water than we give them credit for. I grow a slipper orchid (Paphiopedilum stonei) which comes from similar habitats as northiana in Sarawak. Previously, I used rather fine media which remain excessively moist and the plants dies. Now, I'm having success with coarser media and watering only once every 2 days. Don't worry. The plants will adapt to your conditions sooner or later.
2. Your complex mix seems to utilise a lot of organic matter (dead fer leaves etc). As you plan to grow in a terrarium, I am concerned that the high humidity, coupled with lack of airflow may cause very quick deterioration of the compost and create fungal or bacterial problems. I think that in high humidity situations, a more inorganic compost is the way to go.
Good luck!
