I believe I can answer the first question here. To understand the ways mistreatment would have been considered, its important to remember that not all Greeks were considered equal. The status of those captured would be an important factor in the way one was treated, as would their percieved value to the state. Aditionally, the idea of prisoners of war was not the same then as it is now. Lets keep this in mind moving forward.

If a citizen of a foreign state was captured, there are two main couses of action that were common. First, if they could offer a strategic advantage to the capturing state, they may well be retained as simple prisoners. For example, Athens captured 292 hoplites in the battle of Sphacteria. They retained them as prisoners in an attempt to keep Sparta from invading Attica, threatening to kill them if further incursions were made. This created a dramatic shift in the conduct of war for both sides, and thus measures were taken to keep the prisoners alive. The Spartan prisoners were valuable to the Athenian state because they could be used as a bargaining chip, therefore increasing Athens ability to harry the Spartans and reduce the risk of incurring attacks in their own lands. The return of these troops would be an important factor in the later negotiationsof peace in 521. However, it is important to consider here that it was common for Spartans to travel with enslaved Helots, and that they are not mentioned in Thucydides' description of this battle might suggest they were not kept as prisoners in the same fashion. It is also possible that they were simply included in the number captured, but I think this less likely. Both arguments are simple infrences I'm making and I do not mean to suggest that they are well supported, I only mean to point out that our evidence is of course limited, and that the status of citizenship of soldiers being captured may well effect their treatment.

Beyond this, it was common for captured peoples, soldiers or otherwise, to simply be sold into slavery, or even executed. After successfully besieging Melos, Athens killed all able bodied men of age to fight and inslaved the remaining women in children. This event gives us a number of things to consider. Firstly, it was not wholly inappropriate to execute enemy militant. This was not the first time Athens had considered this course of action, see also the Myteline debate in Thucydides, which suggests that this behavior was not foreign or irregular. To keep prisoners requires the devotion of resources and during a costly war this may not be appealing. Beyond this, it was not uncommon to subject foreign enemies to slavery. While in this circumstance there is no mention of hostile soldiers being reduced to slavery, it is clear this happened. Slaves offered the state feee labor or, through their sale, income, and thes are both important resources, especially during times of war. Sparta's subjugation of all of Messinia creafed for them their entore labor force and allowed, and in many ways required, them to pursue their militaristoc lifestyle. When we consider the frequent revolts undertaken by the Messinians, it seems clear that it was entirely possibel for enemy clmbatants to be enslaved If Sparta had killed all those involved with the uprising, it would lose a great portion of its labor force. Further, slavery was a common punishment for crimes as simple as bankruptcy in Athens, and a common practice across the ancient world. To imagine that captured Helots or other soldiers might not face this fate seems to me unlikely. Greeks did understand that treating their fellow Greeks was important, but profitability or an opportunity to display your cities power were good reasons to instead subject them to harsh punishments.

This is the fist post I've attemptrd to answer on this thread, so I do hope it's satisfactory and helpful.