I have been 8 months with a variety of foot/ankle/heel/calf issues and now with a second physio. I think it's easy to get frustrated - we want the answer and then fix - but getting through cause/symptom is not always easy, and a problem in one part of the body leads on to problems elsewhere as the body moves in a different way to accommodate. It could be well worth following the kinetic chain up into hips/glutes to see if the muscles are firing/tight - I had some glute issues but don't know if cause or symptom...
You say you got back to 18 miles - I question if you went back into training too quick? Did you train each week tas you had trained the week before i.e. 10% rule, or did you train as you thought you could?
In my view a half is good for anyone who hasn't gone long distance before - it gives valuable experience for nutrition, what kit, bike comfort etc and the feel for going long. However, you have ticked that box, so you gave to ask what training benefit it will give.
In my view the impact of a half race can be negative if you taper. I would also suggest that if you treat the race as just a training session, then a 100miles on the bike with a 10min run is a better return for a long days training than doing a half - and much cheaper!. That said I do find a Friel(?) 'long day' training session -1 hour swim, 1 hour rest, 4+ bike, 1 hour rest, 1.5hr run a really useful indicator of how training is progressing.
However, if you want to do a half and it's a fun race then do it - it's all supposed to be for fun anyhow!