Ask away!
1) I let my water go for up to 3 weeks, many hydro growers will change their nutrients out every 2 weeks. But i find that nearing the 4 week mark the plants become noticeably unhappy. so for me, 3 weeks is good.
2)I do lots of checks on the plants, ie visual inspection of the plants, check air temp and RH, but DWC specific checks are PH and PPM. Ph can swing wildly in smaller buckets and needs to be checked daily! if the bucket is too small, you might need to check and adjust your Ph more than once per day.
3)all fully separated, each bucket has its own airstones and pumps. You could connect them to make a huuuge DWC, but maintenance will be a b****. Also my 3 buckets get different nute strengths depending on the phase the plant is in, which i wouldnt be able to do if the res were connected together
4)generally the ambient temp is enough to keep the DWC res in the 16-26 degree zone, so if your room temp control is up to scratch then you should be fine. I battle in winter with res temp and depending on where in SA you stay you might need to raise the water temp in winter. in summer its the opposite, so i run my lights at night and dark cycle during the day to keep my overall temps constant.