According to Clayton (1982) and other studies in the late 80s and early 90s, mitochondrial DNA replication is not restricted to any one phase of the cell cycle (in mice). In some of the studies, it seems they even found mtDNA replication happening more than once in the cell cycle.

Good question!
Mitochondria are bacteria that have given control of their division over to the host cell, meaning that they divide when the cell tells them to, but they still divide like other bacteria, by binary fission. I don't know when the signal to the mitochondria to divide goes out; I'll bet it's in interphase, but it might happen at other times than when the cell is getting ready to divide.

Replication of mitochondrial DNA is not dependent on the cell cycle, but rather the temperature of the cell. Mitochondria do need to replicate their DNA before cell division, however they can do several rounds of replication where as the cell can only do one. For the most part though, they do their replication in the G1 phase or interphase.