If Mertz, Dysart or both are recalled but Hill remains in office, then Hill will appoint new members to replace them, with approval from the remaining city council members. If Hill is recalled and Mertz is not, then Mertz will become mayor because he is the current city council president. He would then have the authority to appoint a replacement council member for himself and for Dysart if she is recalled, with approval from the remaining council members.
If both Hill and Mertz are recalled, then things will get a bit complicated: First, the remaining members of the city council must appoint one of themselves as city council president. That person would immediately become mayor. The new mayor would appoint any replacement city council members needed, with approval from the existing council members. And finally, the council members could again appoint one of themselves as the new council president.