Regarding Pyro Bolts/Fire Arrows shot from Crossbows, I just did a quick test. I am running Version 1.0.1.10
Shooting the fire arrow did NOT give enough illumination to count as Ambush.
Shooting a fire arrow, that then puts target on fire, so that he is literally burning alive. STILL does NOT count as enough for illumination for Ambush.
Subject was in a fairly dark area, so MAYBE it can still work, if the target is already somewhat illuminated, further tests could be done on this. But counting on the fire arrows alone for Ambush, nope!
BUT... dont give up on Ambush just yet. You can use Fire Grenades to illuminate for the Ambush, and this works very well. Just make sure to keep enough distance from the fire, that your own character is still in the dark. In tight spaces you CAN use flares, but doesnt work nearly as well.
My current character is a Crossbow Build which does NOT use traps. 0 points in trap skills. Instead it relies purely on Ambush for doing critical damage. (of course combined with the relevant critical feats). So far I have been having fun with it.
I might do a post describing the build, since it is a bit alternative way of making a Crossbow build, for those who are interested in an alternate type of crossbow build. (though you COULD do combined Ambush and Trapper Crossbow build if you really wanted.)