'I was unable to walk' is not accepted? really? I wouldn't normally mind but having a good answer turned down when you're trying to test out is infuriating.
DL accepted "I was not able to walk for six months", so I think your sentence is definitely correct
Never said "potei" as a navite italian speaker. Just say "ho potuto" instead!
As a native speaker, would you be more likely to say "ho potuto" or "potevo" in this case?
Yes, it sounds a little bit old fashioned...
However, maybe it's not used so much in other parts of Italy, but I personally do use the form "potetti" for the passato remoto.
I'm also a native speaker from the north west and I would never use the passato remoto when speaking, but for example my Tuscan grandfather used it all the time when speaking. It really depends on where in Italy you are :)
You will never use passato remoto in conversation (unless you're in Southern Italy, and even then, you're talking about a totally different dialect). It's reserved for writing, and primarily for literature about a past event. You would rarely use it to write about last Saturday, but you would use it to write about something that happened ten years ago.