Usually it doesn't make any difference as to which switch port the main feed cable is plugged into. You should be able to leave the switch as is. If the previous modem was located downstairs, then the modem was removed and the feed cable from that modem to the switch should also have been removed or connected to something else.
Not sure about the powerline adapters. Do they show up on your bill as a monthly rental item? If not, maybe their yours?? You might have to check with Customer Service or one of the moderators on that one.
As for the single port on the eero, yup, you can do:
Eero ----> switch ----> Port 1 ----> from Eero
Port 2 ----> wallplate port ----> downstairs switch ----> switch ----> all
Port 3 ----> device other ports
Port 4 ----> device