Sorry but I see it as reasonable. The manager couldn't let it be served from the kitchen as it didn't come from the CR. It seems to me that allowing it to be on your table was a bit above and beyond. I don't think most restaurants allow outside food to be brought in.
It sounds like you worked hard to plan a great meal. I'm glad the little girl was so happy with her surprise.
+1 Restaurants always mark up the cost of food. The OP said Chef M wanted $165 for a cake. They bought a similar cake from the Boardwalk Bakery for $120. Sounds like a normal markup. Would the OP have been happy if CM charged $5/ head to keep the cake in their fridge and serve it?
A few restaurants let you bring in a cake you purchased elsewhere. Many (most) won't. I never would have brought an outside cake into any restaurant without calling first. It's still an outside cake, even though it was bought in a Disney bakery.
It's not fair to blame Disney. The OP is the one who spent $120 for a cake without checking first. Letting them keep the cake at the table was more then reasonable. Keeping it in a hotel walk in box was very nice. For some people it's not enough. You want the restaurant to store, present, cut and serve an outside cake? Not reasonable unless you pay for the service. Guests who bring in their own wine pay a corkage fee.
Expecting meat in a buffet to be cooked to order isn't reasonable. Frequently meat in a buffet is more well done then I'd like. Chef M isn't the right restaurant if you're looking for beef cooked to order.
I think the OP had some unreasonable expetations.
Don't blame the Boardwalk Bakery. They had every right to assume you obtained permission from Chef Mickey.