Is a Muslim liable to be executed for a non-believer in cases of wounds and limbs? Does the absence of retribution (Qisas) for a life (Nafs) negate its application in cases less than a life? What is the evidence for this? And is discretionary punishment (Ta'zir) obligatory instead of retribution in this case?

What you perceive as injustice is, in fact, the essence of justice. A Muslim and a disbeliever are not equal, as Allah Almighty says: "Shall We then treat those who believe and do righteous good deeds as those who are Mufsidun (disbelievers and polytheists, who do mischief and crimes and do not believe in Allah's Oneness and in His Messenger Muhammad) on earth? Or shall We treat the Muttaqun (pious - see V.2:2) as the Fujjar (wicked, disbelievers, disobedient to Allah)?"

As for the ruling on retribution (qisas) concerning limbs, the majority of scholars hold that it is subordinate to retribution for lives. It applies between any two individuals between whom retribution for life would apply. One whose killing would not incur capital punishment, their limb is not amputated for another's limb. Thus, a Muslim's limb is not amputated for a disbeliever's, nor a free person's for a slave's. This is the madhhab (definition) of Malik, Shafi'i, Ahmad, and Thawri. As for discretionary punishment (ta'zir), it is established for sins for which no prescribed punishment has been decreed.