Hi R, yes. Just two observations, "subject complement" isn't a common term we would normally use, but it's correct in this example because the subject in this example "I", "states", and this verb is in the complement to the main clause of the sentence.

No, it is not a predicative complement. There is no linking verb. Predicative complements use "linking" verbs to describe the subject of a sentence. Most often, but not always, the linking verb is "to be", as in "I am stating that ..." You cannot use "stating" as a verb having a subject ("I") and an object ("that local production is decreasing"). Only an active verb can have a subject and an object. "Stating" is a participle, not an active verb. It never ever has a subject. Only active verbs can have a subject, as in the following "I state that local production is decreasing". ("State" is an active verb. "I" is its subject. "That..." is its direct object.) Your sentence is grammatical though. In it, "stating..." is an adverbial clause that modifies the main verb "have", clarifying the manner in which "I have replied". The clause does not describe "I". If you want a participle to describe "I", you should place it at the beginning of the sentence: "Feeling tired, I replied to Mr N, stating that ..."