Sarah, I'm so sorry about all of this! I'm tempted to say you teach them a lesson and cancel the AHR at their house! They will lose their deposit money because they were being selfish and imposing their ideas upon you and Kevin.
You should go ahead and look for an alternate location. I'd be happy to help if you want to PM me the details. I have seveal clients based in AZ. Then tell your FILs that no, things are not the same but that was their choice, not yours. You and Kevin apprecaite their generosity in letting you stay in the house and giving you a budget for the wedding. But, now you and Kevin are married adults and need to start a life together and on your terms. If you make a mistake, then you will take responsibility for it. As someone said earlier, if this continues, by the time you have kiddos, it will be out of control!