Adding dynamic second email address to Contact Form plugin
Both, the example from the Contact Form docs

{{ 'me@example.com,me2@example.com'|hash }}


...and the 3rd example snippet in your question

{% set firstEmail = 'me@example.com' %}
{% set emails = firstEmail ~ ',me2@example.com' %}

{{ emails|hash }}


...output exactly the same string for me! So I think you have to search for an answer to your issue elsewhere.