As per my experience
1. Yeah full ammount is paid online. You have to get host 's phone number once u book
2. private room , if there is few rooms in the house then yes there will be other guest in the same house. maybe u can choose whole unit instead if u want the whole house by urself.
3. TRavel credit can be used but there is a minimum spending for each..so check your T& C whats ur minimum spending in order to use ur travel credit.