Short answer: Preview can't be displayed for custom items due to the way they're saved. Long and somewhat technical explanation in case someone is interested: The way it works for normal items is that the preview is drawn from the item list directly, which is very easily accessible. But since custom items are essentially identical to Level saves (meaning, just a bunch of garbled strings that only get decoded on load), previewing would require fully loading the custom item. Parsing, spawning everything, creating the joints, and all that... only to grab a preview picture before deleting everything instantly. It's just not worth the hassle.
 "Mutilate-a-Doll 2"){kind=icon}
No. See FAQ in the description.