An update:
I just talked to the Event Coordinator about a few details.
The cash bar will incur a $100 bartender fee and $35 cashier fee. The bartender fee will be waived if we have $500 in bar sales.
I asked about Disney collecting payments directly instead of me; she said it is typically done through one person in the group but she knows they have had some unique circumstances in the past and she is looking into it.
I also asked about a rain location, since they can't set up the dessert party outdoors there is a >50% chance of rain that evening. They usually make the call in the late afternoon. We will still have the Italy Isola booked for IllumiNations viewing, and they run IllumiNations in the rain anyway, but we'll have to go somewhere else for the desserts. If this happens, then normally the dessert party occurs before IllumiNations, say from 7:00-8:30 pm, then we make our way to the Italy Isola for IllumiNations viewing. As an alternative, we could have the dessert party after Illuminations (this is what we did at my wedding), but it would run pretty late so I figure the early option is probably better unless a majority would prefer it after. The rain location cannot be reserved far in advance (since they can still sell these locations) but we could probably pick a spot a week or so in advance just so we could let everyone know since communicating this info once at WDW would likely be more problematic.