Now I have dwarves dying of thirst on levels 2 and below and refusing to come up to the food stockpile
Did any of them actually die? Are they showing "Thirsty", "Very Thirsty" or "Dehydrated" on the wounds screen? It's normal for dwarves to show "Thirsty" for some time, especially if they're in the middle of a job.
(with plenty of drink in it. I checked using z key)
The total you see under z is for all the booze in all the barrels added together. This can be misleading, especially if you have just a couple barrels with a lot of booze in them, instead of lots of barrels with just a little bit. When a dwarf drinks, she seizes the entire barrel, and holds it until she's done drinking. Nobody else can drink the rest of that booze until she's done.
So if you have 50 units of booze, you might think that's enough for 10 dwarves, but if it's really just 2 barrels of 25 apiece, then only 2 dwarves can drink at a time; the others will have to wait.
I just watched one dwarf come upstairs but the others wont???
If one dwarf can go there, then it's not a pathing/accessibility issue. If you didn't mess with burrows (you would know if you had), then it's not a burrows issue either.
So, I still think the most likely cause is "not enough barrels of booze". It's an easy mistake to make (I made it). Go back to the stockpile and actually look at each thing there using
k. And when in doubt, make more booze. You can't have too much.