Naravno, obavešten sam kako stvari stoje u zvaničnoj istoriji jezika i morfologiji starocrkvenoslovenskog od Vatroslava Jagića na ovamo, ali za nekog ko dovoljno misli svojom glavom dovoljna su i ova tvoja dva citata da se tvoj zaključak, koji ne argumentuješ, isključi. Naime, sve što si napisao do samog zaključka je tačno, ali pre zaključka nedostaje elaboracija koegzistencije dvojstva гражданин, гражданство : горожанин i Рождество, рождение : рожать .
I don't understand what is your question exactly:
1. The word 'гражданин' is attested very early - in the Codex Suprasliensis, in the Codex Assemanius, in Codex Marianus and in Codex Zographensis. A quick look at "Старославянский словарь" of Р. Цейтлин et al. for example will confirm that.
2.That this word is a loan from "church Slavic" can be also confirmed, in Max Vasmer's "Этимологический словарь русского языка" (link).
3. The fact about the duality of some forms in Russian and about the massive influence on South-Slavic (Bulgarian, but also Serbian) on Russian is also well known. Words like гражданин and Рождество have been preserved in Russian because they come/are connected with the Orthodoxy, with the church literature the Russians received and, thus, they have symbolic meaning and belong to the higher end of the linguistic expression.
- Another such example is the pair глава : голова in Russian. Голова is the native Russian word for 'head', whilst глава has higher meanings and is used to denote 'chapter of a book; headman/chief'. (Note that the situation in English is the same! - the native word is 'head', but 'chapter (of a book)' in English comes from the French 'chapitre'.)
- One more example: the pair врата : ворота. Ворота is the native Russian word for 'gate', whilst врата is used in expression like "врата в ад/рай", etc.
In isolated cases the native Russian form has even disappeared, to be replaced by the South-Slavic one: the Russians nowadays say красивый, but 'коросивый' is used very very rarely.