At the end of the day, Regina is a major character who has been with the show since the pilot and is by far one of the most popular. Unless Lana outright refused, then they’re going to do whatever it takes to have her sing. The fact that the Evil Queen’s lair is listed among the scenes to include singing, I think it’s a safe bet that she will sing.
Which is gonna be a horrendous autotuned mess. Lana’s even said she can’t sing so it makes no sense why she would be singing in a musical episode. Unless it’s talking singing. Hopefully it’s just one song.
I hope Emma gets some solos since she’s the titular character of the show.