Fro long rides, I like being clipped in. Partly because it helps when pedalling with a fast cadence, partly for the security (not slipping off in wet weather, or if gears slip, etc).
I use spds, and think that the numb foot problem stems from two problems. One is having shoes too tight - it's tempting to do them up tight so they feel 'solid' on the pedal. The other problem is the small surface area of an spd cleat.
For me, using Shimano spd sandals cured both problems. The sole is very thick and stiff, so I don't feel the spd pressing into the bottom of my foot. The velcro straps are easily adjustable, so I can slacken or tighten them without riding when necessary.
Regarding clipping and unclipping; I turn the spring adjustment screw down to it's lowest setting. If you pedal smoothly, there is no need to turn it up; pulling at a tangent to the axle cannot make the cleat unclip. It will only unclip if you twist your foot.