As I understand you are asking that the Gemara says we cannot exclude Erusin from the Mi'ut which is written in the Parshah of Amah since the Yetzi'ah of Erusin is not complete whereas the Yetzi'ah of Amah is complete. How can the Gemara then say that we exclude Erusin from this Mi'ut since the Na'arah goes out of her father's Reshus with Erusin? She is still partly in her father's Reshus for Hafaras Nedarim!

The answer to this question is that when the Gemara says that the Na'arah does not go out of her father's Reshus with Erusin, this means that she is as much in her father's Reshus after Erusin as she was before Erusin.

The Gemara replies that this is not true because she has gone out of her father's Reshus for Hafaras Nedarim. Even though she is still partly in her father's Reshus, as you point out, this Yetzi'ah is still enough that Erusin should be called a Yetzi'ah from her father's Reshus, similar to Amah.

Best wishes,

Dov Freedman