Wolfgang - As for the negative feedback, there are two problems. You end up with a highly non-linear system whose action at the extremes is hard to predict, and it only takes a small phase-shift to convert negative to positive feedback, with the risk of uncontrollable oscillation - maybe this is what caused (causes) the ice ages in the first place.
More cloud cover causes two things
1. Less heat escapes from the earth because the clouds block the long infra-red
Less heat arrives because some sunlight is reflected back into space.
As far as I know, it is not yet clear which is the more important