I was hoping to get all of these important questions answered before offering something. Since there are no answers then I will comment regardless.
Being calm and assertive is ok but it is not fool proof as every dog is different. Since this is not working for you then I would move to something different but remaining 'calm' which is the key.
Firstly, terrified dogs are my 'thing'. Moving from aggressive to scared and submissive was definately a transition but I have had excellent results. The key to the success I cannot take fully, I must attribute these successes to my little helper, that being a calm, assertive, confident min pin.
If you have a friend or neighbour that has a stable dog that walks well on leash, and is well adjusted to all environments or situations, this will help tremendously. What I recommend is that you map out your route initially with only minor disrupting attractions. Go for a walk with your dog and the other dog, in the same path for 1 week straight and do not deviate from this route. If after the 1 week has passed with positive results the next step is easy. Keep the same route but extend the route to the next obstacle. Maintain this route until you see improvement and then for the next week, extend the walk once again using the same two routes combined with the third 'extension' of the route. Once this is completed, change completely but do the following as close as possible to the opposite end of the route to the beginning (so in other words point Z to A). Ensure that the other dog is with you during this process.
I would also use a halter to maintain full control so that the dog does not pull enough to get loose. I would NOT use a chock collar at all on this dog. He does not require any correction at all, just encouragement. Bring along a favourite toy or treats to encourage him forward when you see he is starting to panic.
Walks should be encouraging for dogs as well as fun. When you feel some tension from him - jog through that area using a 'fun' voice. He will hear you and feel your energy which should snap him out of the 'funk' he is in.
Another great activity for him is obediance courses with other dogs in attendance. He needs a leader that will always encourage him and he needs that security from you that you are there for him.
I have had some very tough cases. Not one dog has been like the other, but what I have learnt is that sometimes I am not enough. I use other dogs that are stable and confident to assist always to get them through their fears.
Please try this. Don't get discouraged as this takes days, and even weeks before you see some real changes. Also, one of the most important things I can tell you is not to anticipate failures. If you anticipate something will go wrong, it usually does. Do not tense up in problematic areas. To assist with this (human mind training), wear some headsets and listen to some music if it helps. This will keep your mind calm and you will then not be tense holding the leash. This helps you also to move forward and it will encourage you to help him move forward as well. Sounds nutty - but for me it works well.
Best of luck with your dog. With time, persistance and optimism - you will see results.