And did he stoop to quibble could tell why (2)
Notice how God has to "stoop" here? The speaker doesn't come out and say anything about weakness, but it's clear that God has dominion over us and we're too powerless to understand why God does what He does. In fact, the speaker doesn't really have an answer at this point either; he just says, God could tell us if He wanted to, but He doesn't. Sounds like we can't do much but accept whatever God gives us.
The little buried mold continues blind (3)
We've already mentioned this line as an example of suffering but it's also a form of weakness. The mold is too weak to change his plight in life. God made the mole how he is, and there's not a whole lot the mole can do about it. It is what it is, right?
Why flesh that that mirrors Him must some day die, (4)
Here's another example of weakness. In short, humans are powerless over death. There's not a whole lot we can do about that, and the speaker is using this as another example of how we're powerless over God's design of life. That is unless you find the fountain of eternal life, in which case, call Shmoop ASAP and let us know where it is, thanks.