Drinking only blood would probably give you iron poisoning. Then there's the question how did the enemies make it all the way to your hole in the ground without any supplies
There's just so many leaks in this plan I don't even know where to begin. Any real desert settlement would invariably be based around an oasis. Just add those into the game and deserts aren't a total waste
Reclaimers and hauling water through the desert are negative sum games. A donkey drinks 1 gallon per day, travels 30 miles per day, and can haul 30 gallons worth of weight. For 20 dwarves (drink 1 gallon per day, reclaim half) living 300 miles into the desert, it would take one donkey every single day to bring enough in, and that's not counting profit or the drivers or anything.
Crude reclaimers could get what, 5 gallons of water from a human (on the high end). (Reclaimers always seemed like a violation of conservation of matter to me. In order for it to work, the water lost to evaporation (sweat, plants) has to be regained somehow. It's like putting a bandage on a splinter when you've got a gunshot wound
The original argument was 'if we make booze require water, and plants require water, nothing could survive in the desert'. Nothing is stopping people from being clever (carving out underground rivers, for example), but I don't think we should go out of our why to make it possible for non-clever people. There's a plate of gold? I'm sure they'll find a way to get water there, but that doesn't mean that we have to magically warp the world to make some special case work.
Hell, people are already building cross world aqueducts...
1st off, based on your math, you're saying a donkey gets 30 miles to the gallon, and can haul 30 gallons on weight. This means a donkeys effective range is 900 miles. Ok, I'll work with that.
Let's say a donkey walks 150 miles out into the desert, dumps off 20 gallons of water, picks up 25gallons worth in weight of whatever awesome resource has attracted the dwarves, and walks 150 miles back.
Now, lets say, one dwarf is leading a herd (is 'herd' the multiple of donkey?) of 50 donkeys back and forth supplying the settlement. That's 1000 gallons of water supplied each trip, and if the thing out in the desert is that impressive, the 25gal worth in weight of it would more than cover everything.
Beyond that, it's the desert. You'd be using camels, which require much less water.
2nd, in
Dune, the desert people wore still-suits. Which collected sweat and moisture from exhalation. Yes, i know its a work of science fiction, but the tech wasn't that advanced, and yes I believe dwarves could make something like that if they were keen.
3rd, death-stills and still-suits to get water wouldn't make it "possible for non-clever people". It would be another resource that has to be monitored and tweaked with to keep your dwarves alive. If you lost your water supply for a season, all your dwarves would die. It would still be a massive challenge.