Cannon roughly means what is considered official story. The definition is likely a bit more wordy, but thats the quick version.

How is exp loss cannon? Creating magical items, like wands and staves, is physically, mentally, and sometimes spiritually taxing. Thats well documented in source books, but the crafter usually recovers (unless they screw up and die or maim themselves in the process), which is why we were going down the path of heavy stamina hit and maybe that str/con/int debuff if you fail to craft. Exp loss is a lasting hit, cannot recover from that with rest or healing, unless it something like drained levels via energy drain or similar spell like effect. All for a well visible sign of stress caused by crafting, but it should be something the player can recover from.
The random spell idea is a neat idea. Still scratching my head on how that could be coded and how it makes sense IC. A wizard of 40+ levels wouldn't prone to let the wrong spell to go off. If this was a skill a wizard got from creation, would make more sense. I could definately see a lvl 12 wizard trying to make a wand slip up and let off a fireball in the room rather than connect a burning hands spell to a wand.
Brew has that explosion effect on failure, damages the character and its equipment... think scribe does as well, but none of my folks have it, so I wouldn't know for sure. Does wonders to show the lethal effects of messing up potion making. Personally, I hate repeating effects so I was suggesting that str/con/int debuff for failure rather than a similar explosion effect. Something new rather than same old, same old. *shrug*
Now, the spell going off in the character's face has merit. Question that pops in my head is what about the non-damaging spells? What if it was an armor spell or ice shield? Perhaps simplify it to one universal effect per school instead? Like all necromancy spell failures casts blind on the crafter.