um, I said I was iffy on allowing Ambush to the 6th foe who has rear facing on you, generally speaking, I wouldn't allow it, for some of the reasons you gave, but mostly because my house rule changes "Must approach undetected" to "The attack must be unexpected.". . .either way, in the book rule you can't do it since the attacker is "Detected" even if they are behind and out of sight, the defender knows they are there. . .per my house rule, the attack is not unexpected either. . .the variation isn't all that signifigant, until you show up at the wedding. . . .
I do allow the "candygram for Mongo" attack ala the wedding buffet stabbing, but I hardly think you could call that attacker having approached "undetected", even if you use duping to appear as a non dangerous waiter, you're approaching "Mis-perceived" not "undetected". If your GM was one of those hard-arsed book GMs, they'd likely say no to the "Could you pass the sugar. . .and die!!" attacks, since you're not "Undetected". . . .as I said above, I was never too happy with that wording in ambush, which is perhaps due to the name of the skill being "Ambush" when in use it seems to actually be "Sneak attack". . .in which case my substitution of "The attack must be unexpected" at my table seems to work out fine. . .
Shrug. Some GMs and Players dislike making Ambush any more broad in use, since it's pretty nasty. 17 ranks of ambush (8th level)hits the sweet spot where you kill/maim on any crit roll of 49 or better when you can fully use it. (How many GMs start artfully creating work arounds to avoid the ambush built PCs from popping all their major foes every session?). . .I know people who think allowing "Candygram for Mongo" is insane and unbalancing. . .I think it makes sense. I think it balances on "what's good for the goose is good for the gander" and it gets used on PCs. . . .It's bad enough getting killed by assassins, but there's a long term social penalty to being that person who always stands in corners, and tenses up to fight mode every time someone walks inside a 10' radius. (in my games, loads of things get balanced on "social" penalties, like having the NPCs not trust you, since you so obviously don't trust them.)