If I buy the worlds master pass will I get all the past missions?

"Since the Missions end each week, the price of the Master Pass will decrease over time."

You cannot go BACK and do past mission, so the master pass will discount once the week is over to compensate for the missions that can no longer be completed.

If you would like more information, check out the Worlds 2017 Missions and Loot here!

As was noted in the announcement, the pass is progressively discounted each week as the past missions have passed.

No. However, the mission's World's Tokens earned gets gradually higher at later weeks. So, you could still possibly earn the Chroma and golden championship ward.