scouseboy - Yeah my post was not very clear, sorry. I should have included that I agree with your scenarios and that I was adding additional ones.
http://www.castlegarden.org/searcher.php And paslwigr - Our disconnect may have finally hit me. The UK has had a centralized marriage registry for a long time. I just learned that fact last week. If an 1847 marriage cannot be found in the UK marriage registry, the marriage did not happen in the UK.
But there has never been a central marriage registry for U.S. When you search marriage records on genealogy websites it seems like you're searching a centralized registry but you are not. In the U.S. each state maintains its own marriage registry but very few states had them prior to about 1900. State registries do not include vital records events that occured prior to the date they enforced their state's vital records registry law.
Michigan's marriage registry starts with 1905 marriages. You found Margaret's marriage in a small local '
historical' registry but it was not Port Huron's registry. Keren's original marriage record very likely is at the St. Clair County Clerk's office but it has not been indexed yet. Their contact info is in a previous post. The reason I had encouraged you to request Keren's original record is because her 1847 record could have a lot more or less information on it than Margaret's even though they were both married in Huron just 5 years apart of so. Recording of details varied widely back then.
Once a state established their central registry, earlier records were deemed 'historical'. Those 'historical' records ended up in hundreds of repositories. That's why familysearch.org, every U.S. library as well as state, county and local historical groups all have their own genealogy finding aids. Here is familysearch.org's finding aid for Michigan.
https://familysearch.org/learn/wiki/en/Michigan_Genealogy.
Familysearch wiki is a good place to go to first before you begin researching in a new region.