To clarify: there isn't always a SME. The person/dept posting the opening gets to decide if they want to use one. If the do, HR does the initial cut based on responses to the questions. Then they pass on the list of those deemed qualified to the SME to look at resumes and determine if those who claimed to be experts on everything really have the skills. Based on the SME review, the qualified list is refined further to get rid of the 'fake' qualifieds.
I think the notifications about being referred happens before going to the SME, though I could be wrong.

I was notified that I had been referred at the same time that the selecting official was notified.

And when are candidates notified re: whether they were referred? Before or after the hiring official? At same time that people who aren't referred are notified?

When does the subject matter expert get involved?

About 3 weeks in my experience. Also depends on whether you have a subject matter expert involved.

How long does it take for HR to return the cert list to the hiring official? I know that OPM wants hiring completed 45 days after the announcement closes but I was interested in particular in this step.