I will probably regret joining this thread...
So, let's try this one more time. In a steady bank at a steady horizontal and vertical airspeed, stall speed = sqrt(sec(angle of bank)) * (straight and level stall speed in the same configuration). What is the missing "mystery factor" that makes this not true?
For "angle of bank" above, substitute "angle off the vertical".
The vertical component of Lift has to match the force of Gravity, otherwise you accelerate downwards. If you tilt Lift off the vertical, you have to increase the load factor to avoid a downward acceleration.
Obviously if you bank the aircraft, Lift comes off the vertical and you have to increase the Load Factor. This is because, for practical purposes, the Lift Vector is at right angles to a line going through the wingtips.
With wings level, and a steady rate of descent, the Lift Vector tilts forward off the vertical. This is because the Lift Vector is
by definition at right angles to the direction of travel though the air. This requires an increase in the Load factor, to avoid downwards acceleration, for exactly the same reason as the bank did.
If you bank and descend, you have a combination of the effects. An exact formula depends on some tedious definitions (the order in which you rotate things makes a difference), so suffice it to say that in a non-accelerating descending turn, the Load Factor has to increase because of both the bank angle and the descent angle.