1) Bachelor's degrees typically take four years. Law degrees take three years. A JD from a part-time law program takes four years.
2) Your undergraduate (bachelor's) degree won't really impact your chances of getting a firm job. Texas A&M doesn't have a law school. UT-Austin is an excellent law school, especially for Texas, If you place in the top 30-40% of your class there, you could probably get a biglaw job. In this economy, it's probably more like 20-30%, but by the time you graduate, who knows. 30-40% is a fairly good estimate.
3) In biglaw, you can expect to work 50-80 hours a week, depending on what cases the firm is handling and how much you're needed. Entry-level salary for biglaw is $145,000-160,000, but may be more or less by the time you graduate law school. The most common salary for new attorneys is actually $50,000/year and 42% of the class of 2008 (law school class) earned between $40-60,000/year. That being said, if you can get into UT-Austin or a top-14 school, you could get the kind of job that pays that well. Not many attorneys make in the $80-100,000/year range - these are some of the least common salaries for new graduates - but it's possible.
4) It's very difficult to transition from a medium-sized firm into biglaw. Most people who get into biglaw either 1) go in straight from law school based on grades and law school prestige or 2) get a prestigious federal clerkship and then go into biglaw. The only way you could really get into biglaw after working for a small or medium-ish firm is if you became an expert in a certain area of law after many years of experience, but this is a much less common route.
There's no legal requirement for you to be an attorney to be a partner, but it would be very unusual for attorneys to make someone a partner who isn't a lawyer. In a law firm, attorneys are revenue generators, and everyone else is overhead.