I'm in a similar position. Due to various reasons my training has not gone to plan. I would say if you want to finish you will be fine but don't try and push yourself for a certain time that you have not trained for. Keep it slow and steady and enjoy the experience, chat to people around , you enjoy the scenery. Things that you cant do if you are focusing on a specific time and can make an event special. You can always enter another one in a few months and do some serious training for it.
Paris is a great marathon. I ran it last year. The course is lovely as you get to see a lot of the famous sights of the city. In answer to your questions, yes you can register in September. You will need to set up an account with ASO at some point before you register but this is easy and free to do but best done before they release the next wave of places. I ran for the British Heart foundation, because I had paid for my own place I just got in touch with them and told them what I wanted to do, I'm sure any charity would be glad of you raising money especially as you have your own place so it does not cost them as much.
I ran my first marathon when my son was one with no running experience till I started training so I know it can be done, you will be fine.
I would advise you have a look at the Paris Marathon forum for 2017 and 16 if it is still about as they contain a lot of useful information and support. I found them a great help. Good luck with the training!
When it came to fuel and hydration I drink water sometimes with an electrolyte tablet in it and use SIS gels and muesli bars to keep me going every 4 miles after the seven mile point. But others on this forum may have better suggestions.
Wise move getting your gait checked.
As booktrunk says just get out there, and dress for the conditions. No excuses - after all it could be raining or snowing on marathon day.