@Robert-Jackson,
My apologies that this has slid by our eyes on this post.
So to answer your question #1. I don't know. The reason is that I've never known any group that I've worked with or otherwise to continue to use the "onmicrosoft.com" past the initial setup and configuration. Most that I've dealt with in the past, getting the email to be seamless handover was key and fundamental instead of waiting. So I'm not sure whether it will work. I'm fairly certain that if it does, rest assured you'll probably get a notification of something like...if you proceed...either you'll see this can be changed later or that is permanent and non-reversible before clicking next.
So on Question #2, It wouldn't affect if people are using the onmicrosoft.com already to access sharepoint or email. If they're not, and currently using your regular domain name, then yes, it could effect them. I do not have any experience like I said of not changing the domain name and go through more setup as the default domain.