d, as MS points out, the vacuum will suck the guts out of the pump. It seems like common sense to me, but everyone has their opinions, be they right or wrong. And, if you KNOW a 30 is passing dirt down to the 10, why not have the 10 in there - maybe even in both.
There is no free ride with filtration. Each time you put something in the line, you make it harder to pull (or push) something through it.
Height from pick up tube, level actually in the diesel tank,length and diameter of the hose, number of bends, valves, etc. are all impacting flow. I think that having an initial water sepaerator and particulate filter is very important — especially with a visual bowl to allow inspection (Racor-type filter). Then having a somewhat finer final filter is standard practice. I’d stick with 30 mic for the initial and then use 10 for final. Unless you have something unusual, that should be the end of it.