How do I fix a tube hat that came out way too long?

You can absolutely unravel from the top, and honestly fuzzy yarn is more forgiving for this than people think. The trick with mohair-ish yarn is to go slow and pull out the bind off stitch by stitch rather than yanking. Once you've got the edge undone, slide the live stitches onto a circular needle a size or two smaller than you knit with, because loose stitches will run away from you fast if the yarn is slippery. Fuzzy yarn actually kind of sticks to itself wich works in your favor here. Do it on a table with decent light so you can see the stitches instead of a tangle of halo, and don't rip more than you need to in one go. For the measuring problem, and this is the part that got you: never measure a tube hat flat. Fold it in half lengthwise and measure from the fold to the edge, then double it. That's your actual on-head length. Or better, once you've got the tube back to a length that seems close, try it on with a bit of elastic or a spare needle holding the live stitches so nothing runs. You'll imediately see how much it slouches and how much to take off. A couple inches of negative ease is the usual target for a fitted beanie, so if your head circumference is known, aim for the tube to be a bit shorter than that before you start any crown decreases. And don't knit 'a bit more for good measure' after it looks right, because that's exactly how you ended up with a gnome situation. That said, if you try it on and love the ridiculousness, ribbing around the brim area and folding it up turns a too-long tube into a deliberate double brim hat, which is warm and looks like you meant it. Ears covered and then some, but on purpose.

Did this exact rescue with a mohair-blend hat last winter and a small crochet hook is what saved my sanity. When I got to the fuzzy part the unknit rows were basially welded together, so I picked the bind off loose with the hook instead of my fingers and it went way faster. The halo hides everything anyway, so any slightly wonky spots disappear once it's back on your head. Also second the giant slouch option, mine lives on a hook by the door and gets compliments constantly.

Honestly the giant slouchy thing can work, I accidentally made one years ago and it's now my designated snow day hat. But if the ears part bugs you, another option is just washing it and wearing it damp over a bowl or a big jar to block it shorter, the weight pulls it down a bit. Worked okay-ish on mine, fuzzy yarns don't love holding a new shape though. Or find someone with a bigger head. My brother got a free hat that day and still brings it up.

I'm in the middle of my first hat too and now I'm paranoid I'm going to end up with the same thing. Mine is on two pointy sticks connected by a string so I assumed measuring against my head was enough, guess not.

Quick thing I'm wondering: when you pull the yarn back out of that fuzzy stuff, does it come off the hat okay and wind back into a usable ball, or does it get all knotted up and wierd? I've got a sweater's worth of the same kind of yarn saved up and I'd hate to ruin it by undoing somethig.

Gotta push back on the 'fuzzy yarn sticks to itself and works in your favor' bit. In my experiance it's the opposite, the halo felts onto neighboring stitches and when you rip, the fuzz locks the row together so you end up picking apart a fuzzy web instead of nice clean loops. I ripped back a mohair-ish scarf once and lost a good chunk of yardage to broken fibers. If the hat fits otherwise I'd honestly just fold the brim up and call it a day rather than fight that yarn twice.

Update: unpicked it and it actually went fine. Used a small crochet hook like the second reply said, the fuzzy rows pulled apart easier than I expected once I stopped yanking. Redid the top about four inches shorter and now it sits right above my ears. Still a bit slouchy but I'm keeping that part. The halo hides the re-knit bits completley.