We have done the Erie Canal - it is unique cruising; an underutilized national treasure. To answer your questions:
1. We found the western portion to be the most scenic, but the entire canal is worthwhile.
2. Yes, you can "do" the entire canal in two weeks (some people who are just passing through have done it in half that time)... BUT, the best part of cruising the Erie Canal is spending some time in the small towns along the way. Great history, friendly people. During the summer, there are farmers markets, festivals, concerts, etc at the different towns. We spent 6 weeks, but that also included a week on the Hudson River and into New York City. We met people along the way who spend their summers cruising the canal, and always find interesting things to see and do.
3. Yes, the Finger Lakes are absolutely worth a visit. One of our favorite towns in that area is Seneca Falls, not on the actual Erie Canal, but on the waterway between Seneca Lake and Cayuga Lake. Lots of wineries in that area. Good restaurants.
4. New York State Canal System Guide.
5. Large fenders (3 per side), rubber gloves, a boat hook for the skipper and first mate, lines to put around the cables. We had "heard" that there were some towns along the western portion where people had their dock lines untied during the night... we never experienced that, but I did make it a point to use dock lines long enough to wrap back to the boat with the bitter end.
6. Not even a little. We had days where we traveled 7 miles, then stopped at the next town. If you are pushing it, you could do lunch in one town and be on the wall of the next town in time for sundowners and supper. The best way to experience the canal is to set your watch back 40 or 50 years. Stop to experience the towns. Museums, a unique local book store, local restaurants (we even found a real Texas BBQ place in one town); we stayed on the wall at one town for a couple extra days to take in concert with a Jimmy Buffett tribute band. If you have bicycles (we took our folders), you can see more of the towns in the same amount of time. In one town, a lady who had been following our blog brought us a bag of "white hots" because I mentioned that I didn't know what those were. Things like that can't happen if you are moving fast.
If you are considering it just for the scenery, you may be disappointed. Don't get me wrong, it is very pretty. Not Pacific Northwest, the coast of Maine, or Lake Powell kinda pretty, but it is more about a look back in history. It is a different kind of cruising than other places we've been... but, it is one of the highlights in our boat travels. Many towns have free or very inexpensive walls where you can tie up. Some charge just for electricity. One town wall even had a washer/dryer (free) for boaters. Easy cruising. We did not use the dinghy. Pull in to the city wall, tie off, enjoy the local flavor.