A few points to answer your questions above:
1) McKinsey pays for your MBA/post-graduation degree if you have received a rating of 'Clear and Accelerating' (2nd highest; usually top 25%) in your last rating cycle
2) They will pay a maximum of 2 years worth of tuition. If you're thinking of law school or PhD, you'll have to cover the duration beyond 2 years. However, McKinsey does cover expenses for books and insurance, and lets you keep your firm laptop as well, which is a very sweet deal. In fact, you get a signing bonus (25-30K) when you re-join.
3) This one is interesting. During your post-grad, if you do your summer internship at McKinsey, they pay the tuition to your school directly. However, if you plan to do your internship elswehere, you still get the tuition money but it's sent to your bank account, implying it is treated as income and is taxed. When you're looking at tuition fees of >$150K, that is pretty significant. (55-60K)
4) One you come back, you're expected to stay for a minimum of 2 years. If you decide to leave after 1 year, you will be liable for 50% of the tuition/expense McKinsey covered
It's truly amazing to see how much McKinsey cares for it's employees. Hope this helps!