For your farming problem, it might be that you left too little water in caverns, so there's no wet soil for dwarves to farm on. Ran into that myself when I dropped water too low.
You can have magma pipe being available in up to every cavern in places without a volcano but high volcanism. So you can have magma pipe surface in cavern 1, and thus have magma above caverns 2 and 3.
Of course, the minimum height from surface to magma sea can be even lower at times (than 20), I think 12 might be possible with 1 cavern.
So I was having fun with toying with the world painter (thx vjek) and.....
Found 3 water springs leading into 1 named brook running through them o.o
It's a long way up, though.
Decided a 4x4 embark to cover that entirely, frozen but it seems one source leads west while two sources make two corners of a triangle with third corner linked to brook going east. The brook's melt acted differently than snow melt, unexpected - parts of it melted later, with murky pools.
I wonder if there's a way to have entire nanofort on brook?
That said, I'm attempting to figure out creating open-air caverns. 400 mountain next to 98 ocean seemed like a promising idea.
Unfortunately, no success - caverns seem to favour lowest surface point rather than highest.
Caves are kinda tiny ^^. Volcanoes work, kinda. Heck, they're probably mandatory, given the
original place I got the idea from.
This was originally land ocean next to mountainside
That said, I didn't bring caverns exactly close to the surface. Oops.
Some more volcano testing:
I put 2 volcanoes in 1 elevation ocean and was surprised to learn that their peaks both ended at 123z, though top layer of magma differed in them (109 and 122).
So, capped the max elevation at 100, min at 98, painted a map of mostly ocean and genned, exploring as an adventurer in elven civ on one of the volcanoes. The elevation was far from flat, though iirc sites tend to mess up landscape (though the volcano did have some missing walls near top, resulting in magma setting the elven forest retreat on fire).
Did 5 more ocean embarks with 400 max elevation(easier to look at in fort mode), but volcanoes this time resting in 98 ocean instead of 1 ocean.
Peak Lava
1 132 118
2 140 138
3 127 124
4 137 135
5 149 144
The islands were bigger as well, though elevations seem to act similarly to 100 grassland/forest. Thought whether the earlier case was perhaps a fluke and did 5 more 1 ocean+volcano tests:
Peak Lava
3 138 122
4 133 131
5 140 130
6 166 158 (Forest retreat, some landscape was in lava. Those wacky elves)
7 143 141
Islands were as small as initially this time. (As an aside, it seems low oceans, even not present in the 16x16 square, tend to push the caverns lower).
Still, it seems volcano height raise is rather less random and more consistent than I thought. I previously got the impression it'd be basically random whether they're a flatland or a several-hundred z tall mountain =D
A smaller question of my own: Sometimes I see things like: two biomes, and they are both Temperate Shrubland Woodlands in Wilderness with Moderate vegetation with same elevation. While I do assume this gives me access to twice as numerous animal pool, is there a way to distinguish the two biomes post-embark?