Flying with a cello?

[skieurope]

As far as I’m aware Southwest allows cellists to buy the instrument a seat

Not anymore. They changed their policy in 2017 IIRC

Please Note: Some musical instruments (e.g. double bass, cello, etc.) cannot be secured in a seat and must be transported as checked baggage.

https://www.southwest.com/html/customer-service/baggage/special-luggage-pol.html