1. It looks like NB calculated the OG on only ~70% efficiency. If you got good conversion efficiency and the 85%+ lauter efficiency you would expect from a batch sparge of a malt bill that large, ~1.070 is reasonable (Edit: Sorry, Tygo, I see you already covered this while I was typing). Also, efficiency measurements are dependent on very accurate gravity and volume measurements, so if you were off a bit on either of these you might get this gravity reading without such excellent efficiency.
That is higher efficiency than most brewers get on their first mash, but it is what I would expect from my brewery. Make sure that you have an accurate way to measure volume and check that your hydrometer reads 1.000 in water.
2. I turn my tun upside down into the compost bin then rinse it out with the hose (I use the handheld shower in the winter). Takes about 2 minutes. What made it so difficult?
3. For calculating mash efficiency (a result of combined conversion efficiency and lauter efficiency), it is best to use the actual potential of the grain, but most brewers don't usually have that information. Most base malts have a potential of 36 points per pound per gallon, after correcting for moisture content. Crystal and roasted malts are usually in the 29-34 range. For an all base malt recipe, 36 is a good value to use for estimating efficiency. For a recipe with a lot of specialty malts, 35 isn't a bad value for an estimate.
Mash Efficiency = (gravity x wort volume) / (grain potential x weight)
(multiply time 100% if you want it in %)
For estimating the expected gravity of your beer, you take the potential of your grain and correct for your efficiency:
Gravity of beer = mash efficiency x (grain potential x weight) / volume
Edit: Kai's site will do a great job giving an understanding of mash efficiency
and an effective approach for troubleshooting mash efficiency