1. "Xia" contains a trace of Xia's soul fragment, which re-fuses through the connection of the power of dreams. After "death", it leaves that trace of soul fragment behind as an anchor and a connection.
2. Regarding Vows and Oaths, although the cheat is massive, it is actually relatively strict. Angel's Breath, a healing Nen ability capable of complete recovery, can generally only be used once a day.
This is completely normal and counts as a relatively harsh equivalent exchange.
Chapter 90