@juliebee I recommend that you attend one of the nights. I've attended several years with adults only. I actually think I see more adults than families there, probably because of the time and also the focus on villains.
You can book the dessert party it you want to sit at a table for a while. The snack boxes are a full meal with meat and cheese and desserts. My husband is gluten free and they make a special box for him which is always excellent. To get a special dietary box, you need to be booked a couple weeks ahead of time and then they call me about a week before to discuss.
You can also be busy the whole time without booking the dessert party.
You don't need to wear a costume. I just wear Halloween themed clothing or haunted Mansion clothes. They sell plenty's of shirts there, too. My husband just wears regular clothes. There are people wearing elaborate costumes and also people just in their normal park clothes.
The treat lines slow down about midway through the event and you can go through as often as you want to. The villains are interactive and very amazing. I do collect candy (which I give away for my own trick or treaters), but sometimes I'll just go through the line to see the villains.
You'll have shorter lines for rides, special merchandise and food and the walk through at villains Grove can be beautiful. I highly recommend that you attend one while there. Are you staying at VGC? Last year, my room overlooked villain's grove and I would have felt like I was missing out if I didn't have tickets.