I know Polish at a nearly-native level and Russian at a basic level.
Old Church Slavonic word meanings sound to me discordant with what I would we expect from those words' morphologies. Some prefixes sound wrongly added to verbs (with regard to the expected meaning), some compound words look over-engineered for everyday things. Some examples (I'm not writing yers):
'vsležeti' - sit down, lay down' 'vsdrjemati' - to fall asleep' 'vslievati' - to pour on.
In those meanings the prefix 'vs' just doesn't fit.
Blagoąhanjie - fragance, Blagodarenjie - thankfulness?
I feel they had to have short, old words for that, as Slavs have now.
It reminds me of Romance - speaking people nowadays when they try to learn Polish. They cannot quite use efficiently the language mechanisms for expressing simple ideas.
Could this be due to the possibility of Cyril and Methodius non being native-speakers of Common Slavic?
EDITED - added examples.
EDITED - the fact of Cyril and Methodius being "highly educated" or "bilingual" does not mean much, imho. Highly educated people can fail to speak correctly in a language that is not their native tongue. Bilingual people - it's up to the question of what 'bilingual' meant in their case.