cerezha,
we had a whole thread on this a while back that you also partook in that I don't want to rehash but to summarize:

sun + water = half life of hours
sun and no water = half live of days

most soil surface has water in it every day except deserts (rain, dew, ambient moisture in the humus, etc).
again the soil gets buried wont photodegrade, but then again it's not directly accessable to the bees (not that any neonic on the surface is that accessible anyway)
Farmers tilling the soil? Depends on if they are going with a 'till' or 'no till' approach. I'm about 20 years removed from my parents farm but I thought most farmers were going with 'no till' these days to minimize erosion (hence the need to eradicate all the existing weeds with herbacide)

top of the grain exposed / bottom of the grain not exposed - not completely accurate, but ground isn't completely flat, nor is all of its components completely opaque. (and most farmers don't farm in sand).

Sergey,
the details you are pointing out seem awfully minor and I don't see how they really add up to a significant issue.