Before addressing your stated concerns I think you need to identify the problems you have had with your prior attempts at constructing a terrarium. Dealing with these issues may be the resolution you need!

Secondly, I would not recommend scorpions in a terrarium designed for jewel orchids. Tree frogs would work. Scorpions really need a much drier habitat.

Charcoal is really an unnecessary additive to a terrarium.
Its primary use, such as in aquarium filters, is in
"sweeting" water passing over it. In a terrarium housing Jewel Orchids you do not want a soggy substrate though you do need a high humidity (70%-80%).

As far as a substrate there are some very interesting and workable ones sold for frog habitats at pet shops. Most are wood base. I think most of those would prove effective.
Mixing in rotting leaf litter is ok but recommend hard leaf litter from trees such as oaks, no aromatic plants leaves.

Planting of Jewel Orchids is probably best with enough spacing to see each plant. These are all sprawlers and will spread across the surface of the substrate if they are happy. Having a rotten log is ok but let the plants grow on their own over it by planting nearby.

Here is a link that might be useful: excellent info on setting up terrarium

my failed attemps were all because i didnt research, thats what i am doing this time. There are a few species of rainforest scorpion that love moist habitats. Actually now you mentioned a licenced animal like a tree frog i might try and get some rainforest dwelling skinks for it eventually.

I hear you on the feeling around bit!!!
Been there and keep finding myself wondering if there is here from time to time!!!!

Here is a link that might be useful: Scorpions

sounds pretty cool what you have in mind; please post a pic when you have it set up!

Kim