sand will only occour in low-vulcanism areas.
so be sure to put a high weight on 0-20 vulcanism for sand. if you also put a high weight on 80-100, you will have even some spots with magma and sand close.
...no. Volcanism does not effect sand.
Sand Deserts appear in areas with 0-9 rain and 0-32 drainage. It's not that hard to make a massive desert world, but you won't have any elves because forests are on the opposite end of the rain/drainage values.
These parameters will give you much more desert areas than usual, while still keeping enough forests to sustain elves (though they still won't always appear):
[WORLD_GEN]
[TITLE:DESERT]
[DIM:257:257]
[END_YEAR:1050]
[BEAST_END_YEAR:300:50]
[REVEAL_ALL_HISTORY:1]
[CULL_HISTORICAL_FIGURES:1]
[ELEVATION:1:400:800:800]
[RAINFALL:0:75:400:400]
[TEMPERATURE:75:75:400:400]
[DRAINAGE:0:55:400:400]
[VOLCANISM:100:100:400:400]
[SAVAGERY:0:100:400:400]
[ELEVATION_FREQUENCY:1:0:0:0:0:0]
[RAIN_FREQUENCY:2:100:3:3:3:0]
[DRAINAGE_FREQUENCY:3:100:6:3:0:0]
[TEMPERATURE_FREQUENCY:1:0:0:0:0:0]
[SAVAGERY_FREQUENCY:3:1:10:100:1000:10000]
[VOLCANISM_FREQUENCY:6:0:0:0:0:10000]
[GOOD_SQ_COUNTS:200:800:1200]
[EVIL_SQ_COUNTS:200:2000:5000]
[PEAK_NUMBER_MIN:0]
[OCEAN_EDGE_MIN:0]
[VOLCANO_MIN:0]
[REGION_COUNTS:SWAMP:0:0:0]
[REGION_COUNTS:DESERT:10:17:0]
[REGION_COUNTS:FOREST:50:5:0]
[REGION_COUNTS:MOUNTAINS:0:0:0]
[REGION_COUNTS:OCEAN:0:0:0]
[REGION_COUNTS:GLACIER:0:0:0]
[REGION_COUNTS:TUNDRA:0:0:0]
[REGION_COUNTS:GRASSLAND:0:0:0]
[REGION_COUNTS:HILLS:0:0:0]
[EROSION_CYCLE_COUNT:250]
[RIVER_MINS:400:400]
[PERIODICALLY_ERODE_EXTREMES:0]
[OROGRAPHIC_PRECIPITATION:1]
[SUBREGION_MAX:2750]
[CAVE_MIN_SIZE:25]
[CAVE_MAX_SIZE:100]
[MOUNTAIN_CAVE_MIN:100]
[NON_MOUNTAIN_CAVE_MIN:200]
[ALL_CAVES_VISIBLE:1]
[SHOW_EMBARK_RIVER:2]
[SHOW_EMBARK_POOL:2]
[SHOW_EMBARK_M_POOL:2]
[SHOW_EMBARK_M_PIPE:2]
[SHOW_EMBARK_CHASM:2]
[SHOW_EMBARK_PIT:2]
[SHOW_EMBARK_OTHER:2]
[SHOW_EMBARK_TUNNEL:2]
[TOTAL_CIV_NUMBER:30]
[TOTAL_CIV_POPULATION:20000]
[PLAYABLE_CIVILIZATION_REQUIRED:1]
[ELEVATION_RANGES:0:0:0]
[RAIN_RANGES:0:0:0]
[DRAINAGE_RANGES:0:0:0]
[SAVAGERY_RANGES:0:0:0]
[VOLCANISM_RANGES:0:0:0]
Keep in mind that due to my personal tastes, they have maximum volcanism, higher savage and evil areas than usual, and fewer good areas. If this is not desired, I can get a more normal-ish parameter set, regarding those.
This parameter set is similar, but with much more deserts and no elves.
[WORLD_GEN]
[TITLE:DESERT_MORE]
[DIM:257:257]
[END_YEAR:1050]
[BEAST_END_YEAR:300:50]
[REVEAL_ALL_HISTORY:1]
[CULL_HISTORICAL_FIGURES:1]
[ELEVATION:1:400:800:800]
[RAINFALL:0:30:400:400]
[TEMPERATURE:75:75:400:400]
[DRAINAGE:0:40:400:400]
[VOLCANISM:100:100:400:400]
[SAVAGERY:0:100:400:400]
[ELEVATION_FREQUENCY:1:0:0:0:0:0]
[RAIN_FREQUENCY:1:0:0:0:0:0]
[DRAINAGE_FREQUENCY:1:0:0:0:0:0]
[TEMPERATURE_FREQUENCY:1:0:0:0:0:0]
[SAVAGERY_FREQUENCY:3:1:10:100:1000:10000]
[VOLCANISM_FREQUENCY:1:0:0:0:0:0]
[GOOD_SQ_COUNTS:200:800:1200]
[EVIL_SQ_COUNTS:200:2000:5000]
[PEAK_NUMBER_MIN:0]
[OCEAN_EDGE_MIN:0]
[VOLCANO_MIN:0]
[REGION_COUNTS:SWAMP:0:0:0]
[REGION_COUNTS:DESERT:0:0:0]
[REGION_COUNTS:FOREST:0:0:0]
[REGION_COUNTS:MOUNTAINS:0:0:0]
[REGION_COUNTS:OCEAN:0:0:0]
[REGION_COUNTS:GLACIER:0:0:0]
[REGION_COUNTS:TUNDRA:0:0:0]
[REGION_COUNTS:GRASSLAND:0:0:0]
[REGION_COUNTS:HILLS:0:0:0]
[EROSION_CYCLE_COUNT:250]
[RIVER_MINS:400:400]
[PERIODICALLY_ERODE_EXTREMES:0]
[OROGRAPHIC_PRECIPITATION:1]
[SUBREGION_MAX:2750]
[CAVE_MIN_SIZE:25]
[CAVE_MAX_SIZE:100]
[MOUNTAIN_CAVE_MIN:100]
[NON_MOUNTAIN_CAVE_MIN:200]
[ALL_CAVES_VISIBLE:1]
[SHOW_EMBARK_RIVER:2]
[SHOW_EMBARK_POOL:2]
[SHOW_EMBARK_M_POOL:2]
[SHOW_EMBARK_M_PIPE:2]
[SHOW_EMBARK_CHASM:2]
[SHOW_EMBARK_PIT:2]
[SHOW_EMBARK_OTHER:2]
[SHOW_EMBARK_TUNNEL:2]
[TOTAL_CIV_NUMBER:30]
[TOTAL_CIV_POPULATION:20000]
[PLAYABLE_CIVILIZATION_REQUIRED:1]
[ELEVATION_RANGES:0:0:0]
[RAIN_RANGES:0:0:0]
[DRAINAGE_RANGES:0:0:0]
[SAVAGERY_RANGES:0:0:0]
[VOLCANISM_RANGES:0:0:0]
EDIT:
stupid me, I didn't read your entire post. Erm... a lot of deserts have trees, I guess. Not a lot of trees, but enough to keep your dwarves in beds. Don't worry about fuel, there's an insane amount of magma with these parameters. Also, the deserts will often occur near