1.) When parents say that there is the option to drop off, do they mean that? Of course, or they wouldn't have specifically said it
Who decides how many of us can drop off because they'd be royally ducked if we all decided to. Is there a rota I don't know about. Not sure there is as it always seems to be the same parents legging it out the door. Whenever I hosted parties, I made sure I had enough adults to look after the number of guests. I also expected other parents to do that. For some that might mean roping in Grandparents or their own siblings or mates, for others that might mean asking some specific other parents to stay.
2.) is booze ever acceptable? Up to the host (and premises). If they choose to offer adult guests a glass of wine or a beer, I wouldn't have an issue with it. Easier than making tea.
3.) is the food for parents too? Again, depends. Generally, no but after the dc have eaten, the hosts would generally not have any issue with adults hoovering up the leftovers if you want food that a bunch of dc have rejected
4.) presents, opened after everyone goes right? To avoid the excruciating thought of the bday kid throwing the book you bought to one side. Yup. Put on one side when guests arrive. Take them home with you
5.) siblings? Are they allowed to? If just one and similar age?
No. Incredibly rude to bring gatecrashers