Questions About Marriage Visitor Visa Application (US Citizen)

I'm trying to fill out the application and I'm confused on a few things.

1. I will be traveling with my stepfather, but refer to him as my father is this alright or should I list him as other when they ask my relation? He is married to my biological mother. I also fill out his information for the part that says list 2 parents information (it does give me the option to only list 1 though). 2. I'm hoping to obtain a multi-entry visa so I can travel to the UK to give notice, return home, travel again within the 6 months to get married and then return home. When I calculate costs should I include both trips or just my first trip to give notice? 3. Can we start booking things like plane tickets before the visa is approved? 4. Are the premium services worth it, what are the typical response times for a decision?

Any help is appreciated or if you can direct me to where I can ask these questions. Thank you!