Good questions. Since my first visit to Sumatra in 2004 I have been fairly obsessed with getting this right, and working with Tony Marsh over the past few months has really brought me a long ways in terms of understanding this process.
As far as equipment goes, the wet-hullers used in Sumatra are said to be modified de-hullers, except with the blades replaced by specially manufactured rubber ones (this is certainly how they look). Rubber parts might certainly help your super breakage issue, although breakage is a big issue for all wet-hullers. Wet-huller operators I have met take great pride in adjusting the machine exactly right, with the smallest possible amount of damage. To answer your numbered questions:
1) fermentation time is all over the place in Sumatra, and ranges from partial with mucilage still attached to fairly complete. Often, I have seen this fermentation take place in a plastic bucket or bag, and washing done in the same bucket or bag in a nearby stream or pool. In the few washing stations that exist, fermentation is complete and coffee fully washed (no attached parchment) with fresh water.
2) Most common drying substrates are concrete patio, asphalt road, or plastic tarps over dirt/gravel. I have never seen coffee dried directly on the ground. I have often seen it turned by rake, this is very very common.
3) The drying percentage is a confusing one. I have heard 40-50% too, but I have also heard 25%. In practice, this varies a lot around Sumatra. I would experiment, I don't think there is any very solid practice. Most operators go by sight anyway, I have heard "you should wet-hull when the bean reaches the color of milk".
4) In my experience, partially-dried wet parchment (anywhere from just-washed to ready to wet-hull) may be bagged, dried a little bit, rebagged, dried again, any number of times between washing and wet-hulling. It may spend hours or days in bags, depending on local practice and market conditions. I don't believe there is any rule of thumb here. By the way, wet parchment is bought and sold by volume measure at this stage, not weight (via a standardized volume measure called a Bambu)
5) Coffee collectors may wet-hull right away (if they feel the coffee is ready) or dry a little while before wet-hulling. Again, the decision of wet-hulling is done by appearance. Just wet-hulled coffee is called "Kopi Labu", or "Pumpkin Coffee", and this coffee is dried, again on concrete, asphalt road, or plastic tarp, until 11-13%.
There is a tremendous amount of local and individual variation on this process, but these are the basics. I have never seen coffee drying directly on the ground, but I 'm sure it happens sometimes. Tony Marsh feels that much of the earthy/mildewy characteristic doesn't come from direct contact with the ground; it instead comes from the fact that immediately after hulling, the kopi labu is very wet and warm, and its protective layers have been stripped away leaving an attractive environment of enzymes, sugars, moisture, etc for ambient microorganisms to set up shop. The enzymatic activity of these microorganisms lend the earthy, forest-floor, even chocolaty characteristics to Sumatran coffee. There is some anecdotal evidence to support this idea, and it seems logical to me. Certainly, the wet-hulling process is responsible for the diminished acidity and increased body of these coffees, since washed coffees from the same areas do not exhibit these characteristics.
Good luck, I'd love to taste the results of your experiments!
Specialty Coffee Association of America