I would analyse "exactly one year after they got married" as [[exactly[one year]] after they got married], with "exactly" modifying the noun "one year" and the whole the ngp "exactly one year" modifying the conjunction "before".
(1) one year is not a noun; it is part of the nominal group; the Numerative of the nominal group is exactly one; the logical structure of the Numerative is b^a —ie exactly modifies one (not one year).
(2) The IFG analysis is exactly one year [[after they got married]], for reasons given previously. So:
exactly one: Numerative
[[after they got married]]: Qualifier