Bart,
Welcome to the 50+ Forum, its always great to have another old guy get onto the sport. Most of the above information and advice is spot on. One thing I will recommend is to read Joe Friel's new book, Fast After 50
Fast After 50: How to Race Strong for the Rest of Your Life: Joe Friel: 9781937715267: Amazon.com: Books, his recommendations on riding and training are some of the beat I have encountered. Several members here have posted that they got a lot of good from this book. Most of his writings have good stuff in them, I like and have used his Cyclist Training Diary, now going on 4 years, and own two others that I keep handy for references.
I am another spinal injury rider, some herniated disks from a fall and spine compaction, at work. No issues with the drop bars and my 2 level fusion and the Ti hardware that came with it. My riding actually helps my back relax, the proper positioning and your bicycle being correctly fitted allow you to have a straight back and let the entire body carry and distribute your weight correctly. My neurosurgeon cautioned me against a recumbent with my injuries, just like not suing a recliner. If the FX 7.7 and the flat bars work for you, stick with it and have a ball, many fast riders love the flat bar set up. Do what works best for you, not others.
Hope that your speed and fitness get to where you are happy, and enjoying the sport. Best wishes on the journey, and stick around the asylum.
Bill