OP, I am very sorry for your experience and I hope that you get some response from Disney. Obviously if you do we would love to hear about how they are handling this type of complaint.
A few things that I want to point out, because hindsight is 20/20, and I always hope that at the very least the readers can learn something from a story like this.
First, I hope in your letter you have primarily focused on the Splash Mountain issue, which seems to really the biggest thing that was out of line that you posted. I know that many are very disappointed about the MK expansion but that has been publicized and there is nothing they can do about it. Change has to happen and they are doing it in the best way they can. Rides and Shows will always be down when you visit. For a long time the Castle was covered in scaffolding that really ruined that magical walk down Main Street and disappointed a lot of people. But they are open 365 days a year so construction projects have to done.
And there are thousands and thousands that echo your disgust with the Dining Plan and the "dumbing down" of the food. There is hope that if enough people let Disney know about it that maybe that can change, so please do let them know, but realize that they will probably not offer much response on that point. They are making money and currently the Pro Dining Plan camp is much bigger than the group that is against it.
Second, if you are uncomfortable with what a CM is telling you, like waving you into a car you don't feel is safe/ready/appropriate then just don't do it. Step back and let those behind you go if you need to. But if your gut is telling you that there is something wrong then it is usually good to listen to that. Waiting a few extra cycles is better than getting yourself into a situation you may regret.
Third and most IMPORTANT:
Deal with any problems IMMEDIATELY. I know you just wanted to get out of there but one of the main things that comes up on these boards over and over is that you need to stand your ground and demand to see a Supervisor or Manager at the time of the incident. Be assertive. Please don't be aggressive or threatening, but don't give up until you have at least talked to the right person. If you aren't getting any satisfaction at the ride then immediately go to the Guest Services office in the park, explain the situation and ask for a manager. They can't help you if they don't even know there is a problem.
I am not criticizing the way you did things, I completely can understand your frame of mind. And if you haven't read here for a long time this is information you don't know. I just hope that your story and this advice will help someone else in the future. Guest Services in the parks seems to do a pretty good job of dealing with legitimate problems. I would like to think that if you had gone to them at the very least you may have gotten a private ride back to your resort and probably some fast passes to use the rest of the day to make up for the lost time. And hopefully even more than that to try to make up for the situation.
Unfortunately there seems to be less success dealing with Resort problems overall. Again, you need to complain of problems immediately and ask to see a Manager. If you just "live with it" or never ask to speak to a manager, and write a letter to complain when you get home you are less likely to get any satisfaction. Document who you talked to and when and what their response was. Also you need to be sure to resolve things in person at the front desk. When you call from your room it is not answered by the front desk like at the Holiday Inn. It is a call center that could be in Minot, ND. Many people don't realize that if they ask for a Supervisor on the phone from the room, you are not getting the Hotel Manager.
I really hope that Disney does get back to you and is able to help restore some of the Magic back to your family.
That it just totally uncalled for. My DH would have been DONE too! He has zero tolerance for this kind of thing. 
. I'm so sorry for you and your family. Sounds just awful, that would probably ruin the trip for me too.
, from the way some people have been reporting the "disney magic HAS left the building"
. Wow, sitting in urine that does not belong to you must have been very un-nerving to say the least
