It seems like a lot of your questions are technical or medical, and though I want to respond, I don't think an English major is the best qualified to help you understand things. So I'll stick to what I actually "know."
First, I, too have a syrinx, about the same diameter as yours (2mm), but mine is in my cervical spine, something like C5-C7. Even there, one of the three NS I consulted said he thought the syrinx might be unrelated, because it was too low. So, yes, your NS is "right", because your syrinx might be unrelated. On the other hand, according to a different NS, the exact mechanism which makes a Chiari descent of the tonsils (herniation) cause a syrinx is unknown. They are trying to understand exactly how the syrinx forms, but don't know. So, as far as I'm concerned, your syrinx might still be caused by Chiari.
Second, I was also told that syrinx can be caused by: tethered spinal cord (which they would have seen in a lumbar MRI if you had one), accident (like a car crash) or a tumor (big scary word, but something my NS wanted to conclusively rule out with a contrast MRI in the months prior to surgery). My gentle advice would be to talk to your NS about whether he thinks there are other possible causes of your own thoracic syrinx(es), and what he might do to rule out these possibilities before the decompression.
Third, do try to find out what he plans to do for your decompression. Just bony decompression, without opening the dura, seems (based on forum posts and a few tidbits picked up here and there) to be associated with revision surgery. It seems like it may not do the trick fully. My own NS says that is why he also "buzzes" the tonsils back during surgery. A scary sounding idea, but I'm about eight weeks post-op and don't seem to feel any affects of that part of the procedure.
Fourth, if the procedure he plans sounds reasonable, don't worry about his skill set. My NS explained that the Chiari procedure is essentially the beginning (opening, cutting the skull to get to the brain, opening the dura) and end (sewing in a patch or sewing up dura, hopefully with no leaks, closing the surgical site) of a tumor operation. And for us, the NS doesn't have to go rooting around in our actual brain looking for tumor bits. So, in some sense, our surgery is technically easier.
Fifth, some specialists can order an EMG, electro myleogram (?), in your legs to test your peripheral nerves for damage. Using this test, my NS could tell that my syrinx had been damaging the peripheral nerves for at least five years, probably ten. So, you would be able to trace your tingling to the syrinx for sure. The real question, though, is if the Chiari surgery will help that syrinx. Try to rule out those other causes, and then consider decompression.
Finally, my personal belief is that earlier surgery is good. My NS told me this week that the longer you have symptoms, the longer it takes your body to heal those symptoms. So, for me, symptomatic (in some sense) for only months, healing is a matter of weeks. If symptoms lasted three years, he offered the example that the symptoms might take eighteen months to resolve. So, personally, once you've determined Chiari surgery might help YOU, I would agree that sooner is better than later.
Good luck! I found this part of the decision process quite difficult, but did get through it. When I went into surgery at the beginning of July, I was VERY certain it was the right decision, and I am very glad I had surgery.