archaeological evidence

Yes, there is ample archaeological evidence for the existance of the glagolithic script. For the existence of cyrillic we also have a lot of evidence. Titmar, Helmold and Saxo Grammaticus all describe a large heathen temple in Slavic Arkona which received gifts for sacrifice from faraway lands (and bits and pieces of which still can be identified on Rügen). Titmar mentions it did have inscriptions but none were discovred so far. From Russia there is a 10th century inscription in the Orkhon script but the language is Khazar and not Slavic. It's on the bottom of a letter in Hebrew written using Hebrew runes. In a 10th century hill grave near Smolensk there was a pot containing an inscription in what might either be Greek or Cyrillic. If it's the former it says "This belongs to Gorun". If it's the latter it says "mustard".