Yes, juefaz, I am a Dr but not an MD, which I think is what you really wanted to know.
To be blunt, I think that you have taken a sort-of shotgun approach to a very real problem.
You are firing all sorts of treatments at your problem, and none of them seem to have worked terribly well so you want to try another one.
So, let me ask ...
Have you ever had a full neurological examination (the sort of thing that takes about an hour)?
Have you had a full spinal MRI?
My take would be that if a chiropractor has not properly identified the cause of your problem(s), and done something about them, then the cause is not skeleto-muscular.
From what you write, a lot of your problem seems to be centered on your dropped foot, and that is most probably a problem with the nerve controlling a muscle in your lower leg. Dropped foot can lead to instability, and fatigue as you tend to do more work with the opposing leg. If the problem does not originate in your brain, then why should reducing a CCSVI condition make any real difference? You don't say where the syrinx is, but to affect your left leg, it would need to be around the T4-T6 vertebrae - in which case it could certainly influence a drop-foot. If you are not carrying a Dx of RRMS, then a DMT is probably not for you - but if a Neurologist has suggested one, you have said no - but then asked for a script for Ampyra, well then I could quite understand that Neurologist, saying that it could not be given.
Maybe you need a fresh start with a different Neurologist. Don't tell the guy what is wrong, set out your mobility and stability issues and let him tell you - but do press for an MRI scan of your spine (maybe your brain as well, but the spine for sure).
If you are with me so far, then look at it like this - if you try Ampyra and it works, then your problem is really neurological, so you want a Neurologist on your case.