Sorry to hear you didn't have a good time at the party.
To answer your questions:
1. Where was I wrong, and what should I have done differently?
Doesn't look to me that you did anything wrong.
2. Is A right? Does everyone who chooses to practice poly take a chance that they might find themselves suddenly in a non-poly arrangement?
Marcus is right, anybody in any relationship takes the chance that their relationship will change at any time. Relationships are sets of agreements. It really sucks that she chose to change the nature of your relationship without negotiating with you. But it happens. Since she doesn't seem interested in negotiating your relationship agreements, your only recourse is to accept her changes, or not accept them and end the relationship.
3. Setting rules and limiting my partner in any way is against every poly bone in my body. However, should I have insisted on us dating only other poly people? Is that limitation okay in your view?
You are within your rights to request any limitations you want in your relationship. It's within her rights to agree to them or not. This is not a limitation I would request of my partner. Instead, I would trust my partner to not do things that hurt me. If that is not important to them, then I would have to reconsider my choice of partners.
4. What would you do in my situation, assuming you want to keep the relationship going.
I would empathize as best as I could with their situation. Express what I desire from them, and see if we are able to come up with a solution that works for both.
Me: 48 - Married, straight, male
Shiela: My wife.
Suzanne: My FWB
Adam: Shiela's LDR