Differential pair output to voltage amplifier transistor
At first, there is a certain asymmetry caused by the base current for Q3. As a result, Vc1 and Vc2 cannot be equal. Of course, Rc must be chosen properly to allow the correct bias point for Q3. 

The one-sided differential gain (referred to the base of Q1) of the main amplifier is Ad=+gm*Rc/2. Thus, the transconductance gm - set by the current source I1 - plays a main role.

It is no surprise that any change of Ic and/or Rc influences the bias point for Q3 and the quiescent voltage Vo. This is a classical "offset" effect. But you should not try to feed back a voltage from the Vo node to the base of Q2. This would result in POSITIVE feedback (instability). But a feedback path to the base of Q1 should work. However, the gain of the whole circuit will be affected, of course.