let me answer your questions in turn:
1) Nope, the number does not have a significance.
2) We don't give a recommendation here, because with the auto zero and temperature compensation, you are well protected against possible temperature related changes in the zero. The risk of a "false positive" - ie you being concerned about a difference between two numbers that was caused by a change in temperature more than anything else, is too big.
3) One unit of offset is usually in the range of 1.5 watts. But again, it doesnt really matter because it gets compensated.
4) As long as you don't have tension on the chain (ie you're coasting and not stopped) you're good
Hope this helps,
official power meter of Movistar Team