I've heard that putting plants in total darkness for up to 48 hours before harvesting stresses the plant, and the plant reacts by increasing resin production. I haven't seen any science that confirms or denies that though, nor have I done that before myself. I will be trying it with my current grow.
I've never put a live plant in a bag, and my initial reaction when I read your post was, "that sounds like a recipe for mold;" plants transpire significantly when it is dark, so unless the bag was somehow lightproof AND breathable, I personally would be concerned about doing that. If more experienced growers think differently, please do call me out on that.
RE the environment, what is interesting in your case is that you have a combination of lower than ideal temperature (leads to slower dry time) and lower than ideal humidity (leads to faster dry time). In theory, they offset one another, but I don't know to what degree, and there is a reason why ideal temp and humidity have been established. The reference to 30% humidity is concerning because at that level, your larger buds could end up being dry on the outside but still moist inside. If you are unable to acquire a grow tent (or some other enclosure) to put in the greenhouse, and use a space heater and humidifier within that to create the right environment, I really don't know what your outcome is going to be. Drying your buds in cardboard boxes or paper bags will create an internal environment that is more humid than the outside, but I don't know how much that will get the humidity up, and you will still have your temp issue. I feel like I remember
@RosterTheCog posting about drying in bags or even a cooler, so maybe Roster can chime in here? You would definitely need to monitor the internal RH and open the box or bag to bring the RH down if it gets too high, but you would also need to do so anyway to exchange the air.
You asked two questions about leaves. The first was pruning. Before answering that, something to keep in mind is that the more intact you leave your plants, the slower the dry time. A larger proportion of water in a plant is stored in the branches and stems, so if you are in an environment that will lead to faster than ideal drying times, you can slow the dry time by cutting the plant and hanging it whole. In the opposite situation, if you need to speed up the dry you would cut and hang individual branches. I'm not sure which situation your humidity and temp will lead to, thus can't suggest which route to take, but something to consider. Now to pruning: some folks trim their buds before (wet trimming) and others do so after (dry trimming). I've heard that wet trimming, followed by another round after the buds are dry, leads to better looking buds ("dispensary quality").
RE the second question about a use for the trichomes on leaves, absolutely. Do some web searches for making kief and/or bubble hash.
