満員 and 空席: a place full of people and an empty seat
Compare a crowded place with seat availability, distinguishing a visibly empty seat from permission to occupy it.
A place full of people
満員, read まんいん, means full of people. 朝の電車は満員でした。 says that the morning train was packed.
会場は満員になりました。 says that the venue filled with people. Read the specified place, such as a train or venue, together with the state.
An empty or available seat
空席, read くうせき, means an empty seat. 後ろの列に空席があります。 says that there are empty seats in the back row.
空席があるか、係員に尋ねました。 says that an attendant was asked whether seats were available. Asking does not establish a positive answer.
An empty seat and permission to use it
この席は空いていますが、予約済みです。 says that the seat is empty but reserved. A seat can be reserved for someone even when nobody is currently sitting there.
On transport with standing passengers, people and seats are different measures. Do not interpret 満員 uniformly as completed seat reservations on every type of transport.
May anyone sit down just because the seat is empty in この席は空いていますが、予約済みです。?
Show the answer and why
That cannot be assumed. The empty seat is reserved, so permission to use it needs separate checking.