Correct in the Tower the external stairs are locked for reentry. They are designated as Emergency Exits so that is the reason.
As
@Jillianna indicated above the elevator delays are primarily in the early evening when guests are exiting from the MK. Many guests stop at the Contemporary for food or to look around the resort. There can also be slight delays in the morning when it sometimes seems like everyone is trying to head out at the same time.
It is not like you have a 20 minute wait, you just can't walk up & push the button since the elevator capacity is reduced due to social distancing. Once the Monorails stop for the evening the elevators are not an issue.
Dave