How much was Old Church Slavonic edited by Constantine and Methodius? And what modified more: The Old Church Slavonic when people in Bohemia started to write with it, or the slavic dialect they used before?
Yes, Old Church Slavonic (OCS) was an artificial language, but just in a way.
Firstly, in the 9th century, when Cyrill and Methodius devised the OCS, all the Slavic languages and dialects were so close, that they were closer to one another than the modern English dialects inside England. Secondly, Cyrill and Methodius did not create any new phonological system, they used their native Pannonian one, with all its tendencies.
What Cyrill and Methodius added to their contemporary Bulgarian-Macedonian dialect spoken in Thessaloniki was the Greek-Bible-style syntax and the Greek-style lexicon (lots of Greek words were borrowed, lots of Greek words were calqued). That's all, the job of Cyrill and Methodius was just to shape the Slavic lexicon and syntax to fit the Christian literature standards, nothing else.
And naturally, OCS had nothing to do with the then dialects of Bohemia or Moravia which were Western Slavic dialects, OSC has always been a South Slavic language.