All problems with "the Design Hypothesis" may be thus handwaved away. "We cannot second-guess the Designer."
However, as to design in general, once you understand the purpose of something and how it works it is easy for an intelligent designer adapt it to other situations.
"... Unless it's convenient to second-guess the Designer."
The "Designer" of ID does not act like a human designer. He is inefficient, clumsy, kloodgy, doesn't modularize or share design where it would be expected, but does borrow design where it is unexpected. When he made the bat, he apparently didn't borrow from the bird at all but from the tree-dwelling insectivores. Those are the parts he adapted. Odd. When he made the whales, he didn't borrow from the fish but from the land mammals. Again, odd. When he made the thylacine, he didn't borrow so much from the wolf as from the kangaroo, or something like it. Odd.
But we're not allowed to count any of that against ID, since we cannot know what this Designer was up to. Except when it's "easy." This isn't science. It's just an adaptation of Genesis via creationist pamphlets via rewriting to omit reference to the Christian/Jewish God. It predicts nothing, and tells us nothing about the history of life on Earth.
"... Unless it's convenient to second-guess the Designer."
No, I am not second guessing anything. This is a scientific discussion not a theological one and I am explaining the situation on a scientific basis which is supposedly what the theory of evolution is about.
The "Designer" of ID does not act like a human designer.
'Should' is not a scientific term nor is it an objective criteria for anything. It is totally rhetorical and therefore does not address the scientific questions which I have delineated.
Further, for you to show that 'convergent evolution' is anything more than rhetorical nonsense you have to show, not that species which are totally unrelated have similar features, but that species which are totally unrelated inherited those features. Such you will claim is a non-sensical request, my reply to such an objection has already been given - the concept of convergent evolution is nonsensical therefore any proof of it will have to be nonsensical in itself and not in accordance with scientific fact.
When he made the bat, he apparently didn't borrow from the bird
The above statement is scientifically false:
Bat bones tend to be light and slender (which accounts in part for the sparse fossil record of bats). As in birds, bats have some reduced bones, the radius and fibula are shortened and thin. Bats, like birds, also have fused cranial bones for additional lightness. The arrangement of the muscles is also designed for lightness and for flight.
From: Amazing Creatures, an article which evolutionists could not refute and therefore abused to force its being pulled from FR.