It’s more interesting how it appears she keeps expecting you to compromise. It’s okay to just let her be unhappy about it. She’s not willing to meet you half way. If this was truly an issue she has 9 months to figure it out. It wasn’t really important then.

Two last names are not an issue. Most people don’t even reference their middle names beyond an initial.

For whatever reason she’s becoming unhappy. This is no guarantee to fix it or benefit to the child.

But you have to decide your feelings. It’s really up to you and no one else can really tell you.