Here's some quick responses:
1. I don't worry about the brief time that the fermenter is open. First, its brief. Second, there's a layer of CO2 on top of the beer, protecting it from things. But, mainly, its brief.
2. I recently got a wine/beer thief - http://cgi.ebay.com/Sample-Thief-for-Beer-or-Wine-Making_W0QQitemZ380116582439QQcmdZViewItem - and its easy. Otherwise, you can just siphon some beer out of your fermenter. Either way, just sanitize like you normally do.
3. For most of my beers, I don't monitor the beer's gravity as closely as some do. I tend to take an original gravity reading and then a final gravity reading, before bottling - usually four or more weeks after brewing. If there appears to be a problem with the fermentation, or its an unusual fermentation, I may take a reading or two in addition to those, just to track progress, but for most beers, I don't see the need.
Hope this helps.
edit: I didn't buy the beer thief from the ebay seller above, just posted the link to explain what I was referring to - bought mine at my LHBS