This cruel universe I love so dearly

[Ryft]

1. Why do they "have the obligation to explain" the intentional designer's purpose in these circumstances?

I didn't say 'purpose' but stated that "those who claim intended design have the obligation to explain why this cruelty is intended in the first place". This is so because theirs is the claim to know the intentions of the supposed designer. Surely this means that they can answer the simple question whether its design was intended to have this cruel consequences on humans.

First, to ask that someone explain why X is "intended" is asking them to explain its purpose, if "intention" has any teleological implications at all. Second, you are mistaken: Christian theology does not claim "to know the intentions" of God with respect to human "ruin, injury, pain, misfortune or destruction to/of others." If Smith tells Jones that he has a purpose behind something, it does not follow that Jones knows what that purpose is. Third, pointing to someone's ability to explain does not somehow establish their obligation to explain. Why do you assert they have this obligation?

2. What conclusion can be drawn about an intentional designer if those with this supposed obligation do not themselves know what its purpose is?

The primary conclusion is about the truth value of these asserters of intended design. It shows their understanding of design is too poor to credit the claim. ... [Y]ou might conclude that they are full of shit for worshipping a designer God whose moral they haven't checked. They could be worshipping the devil in disguise.

"These asserters ... their understanding ... they are full of shit ... they haven't checked ..." In short, no conclusion about an intentional designer can be drawn. If those with this supposed obligation do not themselves know what an intentional designer's purpose is, you can draw a conclusion only about them.

3. What if the purpose in these circumstances is greater net moral good (e.g., compassion is a virtue but it presupposes suffering, courage is a virtue but it presupposes fear, etc.)?

Have you any indications for that, for at this moment it is only wild speculation?

First, I do not need to establish that this is in fact the case, only that it is logically possible. For if it is possible in a logical way (i.e., the premises do not self-contradict), then any argument for a logical contradiction vanishes. Second, nevertheless there are indications for this; e.g., "You intended to harm me, but God intended it for good to accomplish what is now being done, the saving of many lives" (Gen. 50:20).

I could also speculate that the purpose is net moral evil ...

You could, but to be a valid and legitimate criticism of Christian theology, you would have to argue in favour of that speculation from Scriptures or orthodox Christian theology. Otherwise it would amount to nothing other than a competing view that leaves the Christian view untouched.