Re screen top and 70-80% humidity, that does not work unless you live in an area where your indoor humidity is 70-80%. If you need to elevate the humidity, as I presume is the case, you will need a fairly tightly fitting transparent lid, glass or acrylic. A small slit possibly 3-5% of the total top area will be open to allow for air exchange.
Re spider mites, look at the stickies in the terrarium section, and check out the dry ice eradication method. I have used it, and it works wonders.
Waterfall vs middle pond/stream, that is just a question of optical design. Whatever you like will do fine. There's no right or wrong.
For plants, see the sticky post on plants.
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