I think my maths are wrong on the depth of field stuff. But just looking at a 50 lens depth of field calculator, and given an enlargement distance of about 40cm, you do indeed have roughly 1.5cm in front and behind the focal plane to play with. This then doubles the angles I was talking about. So...your enlarger would have to be pretty badly out of whack for this to be a problem. Of course, any misalignment does introduce a technical reduction in contrast, and the circle of confusion limits are really when the MTF drops to 10%, which is pretty dire, so backing this angle limit off 50% would give a 50% MTF etc. Just print and be done with it I guess! Prints are coming out fine, so its not really a problem.
Regarding my problem with 4s steps, there is lamp cool down and warm up at these times...anyone using a PCS 2000 should try to avoid sub 10s exposures I reckon. Just backing the lens off to f11 or f16 obviously extends the time. The problem with dropping the green channel back, for example, is that this alters the contrast, as its not possible to entirely shut down either blue or green, without physically putting a green filter at the end of the lens. Some light comes through, even when the channels are switched right down.
A Stouffer step wedge has just come through which ought to assist in working out what contrast range I can get in the limit, full green, or full blue, and I can work from there.