Because the headline is one adult with nothing added to it. Your total covers everyone travelling, at child rates where they apply, plus a kayak if you ticked it and a surcharge if your hotel sits outside the normal pickup zones. Whatever it comes to, the quote has to show you that figure before you pay.
Depends on the boat. Some have it built into the price and you never think about it again. On others the crew takes it on the day, so bring cash. The park fee column tells you which one you are looking at.
Sometimes, if you are lucky and it is quiet. In December and January you are far more likely to find the decent boats already full. Booking ahead at least fixes the operator, the date and what is included, in writing.