-If there's no moon, the exposure is right. I've had exposures of buildings under streetlights last several seconds with super-fast ISOs (like 125,000)... add to that a slow setting and bupkus for available light, you're looking at many hours (the difference between 2 hours and 8 hours is what, 2 stops?).
-Only one pic has too much water blur, I think, but I am far from an expert. The rest look fine.
-Not sure about that one, either... Maybe closer to the equator than myself (about 42N)?
You're worried about focusing at an object billions of miles away? You can use the planet's orbit of the sun as a rangefinder and still not get an accurate measurement of distance (look up the definition of parsec). SLR's have been considered superior for decades (precise framing, cheaper bodies), especially if you put it on a telescope at some point, with which many "star-trail" fanatics eventually get involved.