One disadvantage to that in my view is that your solution is now dependent on Synapse.
Sure, but the methodology I developed can easily be ported over to any other platform if needs be. That actually makes me more confident on the solution I arrived at.
This also illustrates why REST has displaced SOAP (...) all of the services I use (over a dozen) now provide REST as an option
Again, I agree. Though I find the structure of SOAP more appealing, the usability and platform independence of REST is way higher. If the web services in question were mine to develop, I would definitely give that a lot of thought - but, as I mentioned before, in this particular case I'm only consuming services provided by my customer, and I don't really have any say on the matter.
Thanks for all your input, you have added lots to my personal knowledge. Best regards to you!