Hmm, yes I see what you mean about digging still being too easy - I like embarking in thick soil lands in vanilla DF. This carried right through for me as I found a sand layer and just dug out huge swaths of the 'soil' to get my infrastructure up. Having all soil have the risk of having dig-interruption 'drops' sounds fair - certainly it would encourage me to look for less soil so I can get to ore faster. Though since you want players to build above ground forts, I take it there's an assumption that players will always want to embark in high vegetation (shrubs and trees) areas?
On the note of resources, have you considered putting in a reaction to make many stacks of pellets/bolts at once, just as bullets/shells come in bulk? Currently a whole log has to disappear to get 25 wooden pellets/bolts. In my last Corrosion fort I had some 30+ riflemen who went through ammo very fast on archery ranges. Building lots of air rifles or crossbows and using wooden ammo to train up rifle skill sounds like the economic way to train an army in this mod but current pellet/bolt production is very resource inefficient.
By the way, is it intended that shotgun shells make for better ammo capacity? Shells are produced in stacks of 15 which means a soldier will pack 2 stacks of 15 into his quiver whereas can only fit one stack of 25 bullets. Given shotguns hit harder than rifles, doesn't that mean the only reason to use rifles is the bayonet (and if you're resource-limited)?
If I recall right people will try to get to 30 without going over, I've seen my dwarves in vanilla haul around 25 bolts plus a stack of bone bolts.
Hmm, I thought the limit was up to 40 or so, but I never checked carefully. I wish quiver sizes could be modded, individually per ammo item would be better. Alas, we are stuck with one bullet being the exact same size as a shotgun shell in quiver-space.
This is kind of a nooby question but where do you make screwdrivers and screws?
Screwdrivers get forged at a weaponsmith's forge. Screws get made at the new "blacksmith's workshop" building.