That’s too bad that SW changed their policy because they used to be pretty accommodating. Because they don’t assign seats until boarding at the gate they would allow someone who had bought a seat for a cello to be in the first group to board and make sure they got one of the front seats.
My S has also flown Delta, United and Air Canada with a seat purchased for a cello and those flights went well. It is a good idea to carry a copy of the airlines policy with you and he has always booked his flights by calling the airline directly so that they can make sure to assign him the correct seats.