1. leave it blank
2. visa number on the passport
3. W7 form can only ted when you file the 1040 form of the year. You should claim your mother as dependent in 2005. Since it is 2006-2007, not sure if you can still do that for your mother. Better consult the agency of IRS face to face. However, you definitely can do that for your inlaw as long as the residential test (183 days) is met.
4. To avoid unnecessary trouble, you'd better sign that for your mother (simulate her handwriting:)