Level 70 corresponds to distinguished engineers, very few engineers have that level at MS (and Google etc). You don't get hired at that level unless you're already a computer science celebrity.

And yes you have to stick around because the 310k of stock will likely come with a vesting schedule, usually 4 years, i.e. you get 1/4 after one year and the rest gradually over the course of the 3 remaining years.

It's bigger than the base pay because stocks is basically the company printing money. Also it's only an initial grant, while the salary has to be paid every years until the contract is terminated.