For very early morning flights (5:00 AM- ish), what is the earliest someone can enter MCO airport (not necessarily checking-in, but simply enter the airport and maybe dine/shop)?
The airport is open 24 hours. Some restaurants open at 4 AM- like the kind of places you would get coffee and baked items or breakfast sandwiches. Ditto the newsstand that is more of a gift and convenience shop. Vending machines 24/7 as is the McD’s. So if that meets your needs for food there are some options. https://orlandoairports.net/getting-around-mco/shops-restaurants/
According to the Orlando Airport FAQ page, the airport is open 24 hours:
How early does the airport open?
The airport is open 24 hours per day; however, each airline sets its own hours of operation which is dictated by its flight schedule, therefore please contact your airline directly to determine how early you may check-in for a flight.