<p>Here's an explanation I gave for the first question about a week and a half ago:
"The villagers found the visitors as fascinating as their customs were mystifying."</p>
<p>The first "as" is an adverb modifying "fascinating." It means equally, or similarly.
The second "as" is a conjunction. It means "to the same degree that."</p>
<p>If you plug in the definitions ("The villagers found the visitors similarly fascinating to the same degree that their customs were mystifying"), it sounds awkward and redundant. The point is that you need an adverb to modify the first component ("the villagers found the visitors . . . fascinating") and a conjunction to connect it to the second component ("their customs were mystifying"). The conjunction--the second "as"--establishes correlation between how much the villagers found the visitors fascinating and how much they found their customs were mystifying. The "found" is implied in the second component: "The villagers found the visitors as fascinating as [they found] their customs were mystifying." Or maybe it isn't implied, and "their customs were mystifying" is all there is to it. It makes sense in the passive voice as well. I'm not sure.</p>
<p>"He is as cold as ice [is]."</p>
<p>It is the same thing here. He is similarly cold; i.e., he is cold to the same degree that ice is cold. The "is" is implied at the end.</p>
<p>Other similar structures:
"You are as good as you think you are."
"It is as cold today as it was two days ago."
"You have accomplished as much as you could have."
"That is untrue as far as I can tell." (This is idiomatic I think. But it's good to make association with familiar phrases.)
<p>Regarding the second question:
How is "different from" an idiom? Just because you see "different" doesn't mean you have to use "from." This precisely means that "different from" is not an idiom (I rhymed). "X is different from Y" means that X and Y are different. "X for children is no different from X for young adults" is the same structure, but since the distinction is between whether X is for children or for young adults and not between actual structures X and Y, you simply say, "X is no different for children than for young adults." I hope that makes sense to you.</p>