Isn't the whole point of hostages or human shields is that you're supposed to not kill them? I think it's generally seen as bad if you kill civilians when taking out "bad guys".
I do not understand why this became such a huge argument for why the Palestinian casualties are so high.
Qassams are a few dozen kilo in weight, have limited reach and explosive capacities and their range is determined by their fuel amount, its quality and the altitude from which they are started (also their quality in terms of aerodynamics).
Fadschrs on the other hand weigh under a ton and have a reach of around 70kms, when used from higher places it can increase a bit too reach 80+km (eg Jerusalem). The speed is below 5km/s,so if you would be at the height of a four story building at 50m you would gain 3 seconds of flight (so at max 15km extra).
Every meter higher they are fired from they fly much further. This means, yes, ever story helps the reach (and drastically reduces the chance for them to hit buildings in Gaza as - at least four qassams - the solid fuel is very dangerous and not seldom explodes to early or has the wrong mix and thus after starting falls close to the place they are fired from.
To prepare 2000 (as were used within the last couple of days) on fields would mean Israeli intelligence would likely notice the preparations earlier and you would have smaller reach and would be exposed (cause outside Gaza proper the aren't cellars, nor would the IDF warn in some cases that they are going to strike. Which they do sometimes, but not always (eg when they declare a location to be a military Hamas base contrasted with a civilian complex used to fire rockets from OR when they fuck around / 'precision hit' neighbouring buildings instead their targets).