Your confusion is understandable. The antecedent of quem is Albula. Ovid may simply be treating it as a masculine proper noun. On the other hand, he may be taking the antecedent to be not strictly Albula but a generalised amnis/fluvius, or, more likely, he has attracted the relative to the gender of the complement Tiberim, which again is masculine (usually!).
The subject of reddidit is the noun phrase mersus Tiberinus (“the drowned Tiberinus”).
Victor covers everything. OLD notes Ovid has Albula as a masculine noun, otherwise it’s feminine. Victor omnia narrat. OLD dicit Ovidium Albulam nomen masculinum habere, aliter alibi Albulam fluvium femininum esse
Rivers were generally represented as male gods. Even if Albula was feminine before Tiberinus drowned in her, she was transgendered when s/he became the Tiber. Without fluvius or amnis, the passage from the Fasti seems jarring, but this may be an example of Ovidian verbal wit.