Only if: Como utilizar

Hey, guys! Good morning!

Then, I was studying invertion and I have a question about the phrases that use "only if".

When must they be inverted?

For example, I have two sentences in the exercises where the book says they're wrong:

1 - Only if it's convenient she will come.
2 - Only if he wasn't available did she come in.

The reason the book uses to say these are wrong is that the phrases are unnatural. I couldn't understand the general rule, so if someone could help me, I'd be really glad!

Thank you for the help!