a big deal for you guys.
Telling a large institution that you are planning on doing one thing and they tell you to do something to verify that and you just ignore their request for 6 months while you do something different seems a little crazy to me. But again that is just me.
Remember that BYU is a massive institution with probably 5,000-10,000 students that leave for missions annually. They have to figure out who is coming when and have a finite amount of space every semester. From their standpoint I bet they can't imagine a child and his parents who get accepted to the institution and then tell them he is going to defer to go on a mission. So they send out information on what they need to do that and then 6 months goes by and the parent/student call up and want to completely change what they told you because their circumstance changed 6 months ago.
Based on what happened I just can't believe you are putting the blame at their feet. They did what they told you they were going to do and you ignored them for 6 months and you are now mad that you can't just change things to how you want them to go.