What writing system, if any, did speakers of Old Polish use for the centuries preceding the adoption of the Latin alphabet?

Answer 1

It was always Polish or high Polish, based on Latin and languages of our neighbors:. From Wikipidia:

Earliest written sentence

"Day, ut ia pobrusa, a ti poziwai", highlighted in red

The Book of Henryków (Polish: Księga henrykowska, Latin: Liber fundationis claustri Sancte Marie Virginis in Heinrichau), contains the earliest known sentence written in the Polish language: Day, ut ia pobrusa, a ti poziwai (pronounced originally as: Daj, uć ja pobrusza, a ti pocziwaj, modern Polish: Daj, niech ja pomielę, a ty odpoczywaj or Pozwól, że ja będę mielił, a ty odpocznij, English: Let me grind, while you take a rest), written around 1270.

The medieval recorder of the phrase, the Cistercian monk Peter of the Henryków monastery, noted that Hoc est in polonico ("This is in Polish").^[3][4][5]