Does the brown discharge that accompanies a threatened miscarriage necessitate Ghusl (ritual bath) or only Wudu (ritual ablution), and what is the ruling on prayer in its presence?

A pregnant woman does not menstruate. The discharge you are seeing is not menses. You must perform ablution (Wudu') and pray. If the discharge is continuous, perform ablution after the time for prayer begins, and pray your obligatory and supererogatory prayers with that ablution. You must make up the prayers you missed, as they are a debt upon you.