I've had a look at your model, it's quite big! You have hit the limitation of model builder and it is creaking under a complex interface. When you change anything a whole load of validation is run behind the scenes which you have no control over.
There is really only 1 option here and that is to convert the entire model into python code, I suspect it will also run faster and you can organize it to be much more efficient.
With python you have two options:
- Develop it as a python script and then wire up an interface to it. This would be the easiest approach but could potentially lead to the same slow down.
- Develop it as a python addin toolbox and define a new interface for it. An obvious one is as you have a lot of repetition for similar like inputs a value table would be a better choice. Your user would complete the table then it is just ONE parameter that needs validating not the dozen you have now.
If you are not comfortable in programming in python then there are many developers out there who could do this for you but obviously for a fee.