When one records a marriage event, it would be better if a category of "Marriage Registration" be made available to select. Often the actual marriage record is not available.
Thinking about exactly what evidence you have is good - no question about that. But my thought here is that there can't be a "Marriage Registration" event without there being a "Marriage" event in the first place. You might not have a direct record of the Marriage event itself but surely you have an indirect record of the Marriage?
So if it were me, I'd record a Marriage event, add the source for the Marriage Registration, and then the only question in my head is what date to give the marriage event. All the Marriage Registrations that I've seen give the date of the actual Marriage, so I'd put that in - but there may be Marriage Registrations that don't record the Marriage date, so in such a case I'd set the Marriage event date to "Before dd/mm/yyyy" where the dd/mm/yyyy represents the date of registration.
Is there a hole in my argument?