The neurologist is of course the expert but this treatment approach just seems excessive to me. Unless the neurologist actually saw the scratching and assessed it as actually being a reaction to having a syrinx and thinks the stiffness is painful and definitely caused by the syrinx (both of which seem unlikely with a tiny syrinx, from all the conversations I've had in the last couple of years with researchers) I don't understand why she is even on a painkiller. Really, if all of us on the board MRId tomorrow probably a third or more might MRI with what you are seeing and most of those would have no symptoms at all. That is why I wouldn't recommend MRIing without understanding the broader context of what is being seen by researchers -- eg a high rate of affectedness but a lower rate of symptoms -- and understanding the various approaches to treatment, IF NEEDED.
If the neurologist truly believes Star is scratching from SM and has seen this for herself to separate out what is normal scratching, then perhaps she needs medications. Yet most dogs would simply be put on one of the CSF drugs and might never, ever need something like gabapentin. It also sounds like you are giving 100mg three times a day -- it took 4 years before Leo was on that high a dose, just to give some context, and he is showing a lot of definite symtpoms. A dog that just scratches a few times a day for 5 seconds or under is not scatching due to SM. SM dogs scratch for long, long periods and cannot be distracted from scratching.
If you go to www.smcavalier.com, all the treatment approaches are explained, all the medications are explained, and you can see videos of typical SM scratching, which does very much stand out from normal dog scratching. My clear dogs would scratch many times a day for a few seconds at a time, just for context.
I just do not believe a valuable painkiller that is known to lose effectiveness over time should be wasted by being used too early for a dog that isn't having any problems.
Keep in mind that dogs with the scan you received (and if the scratching is not SM scratching) would still be considered fine for breeding under the researcher recommendations -- this is not a bad scan at all from what you have described, and is a very minor syrinx. I just think you really need to clarify whether you re seeing SM scratching so as not to give medications that are not necessary.