Can anybody give me a definite answer on this? In the official xmls there are neither any level 15 items (which would be something you'd expect to have if actual level were used), nor any level 0 items {except for puffballs} (Which would be expected to exist if level minus one were used). This is a puzzling state of affairs.
The item levels need not to be exact as every level spawns items with a range of item levels, meaning that item level modifies the chances of given items spawning on certain levels but it's not said that only level 15 items can spawn on the 15th floor. And since I'm already at it, the actual level is used. Puffballs are an exception of sorts which is why their item level is 0 - I don't remember the exact reason now but there was a mention of that.
No, but the level stat does determine where they primarily show up. Thanks Well puffballs have special="1" anyway so it doesn't matter what level they're set to because they won't spawn normally anyway.