Do you have a sag in the roof? Houses have been built for thousands of years without a ridge. Trusses do not use a ridge. It held up the weight of a slate roof for how many years? It must be pre 1940's. The 12 12 pitch helped. when a roof shows evidence of thrust, you usually will see the walls bowed out too. You could add the ties up higher if you feel it needs them, it isn't going to hurt anything. If the town wants an engineer to stamp whatever you are doing you will probably have to play their game. If 3 engineers all gave you a different plan, how can you believe any of them? get an engineer to stamp their stamp?? Hope everyone else has better information to give you than I have.