MacGuffin is a random item that the characters are searching for. Typically, you can swap out anything for the MacGuffin and the plot won't really change. But I know what you meant - they were foreshadowing. But it's kind of a big plot hole - if Pym Particles reduce the distance between atoms, the weight of the object should be unchanged. You wouldn't be carrying around 10-ton tank in your pocket.
I think you're right about casting Janet Van Dyne. I expect Hank Pym will attempt to save his wife from the Quantum Zone in Ant-Man 2, now that he's been shown a way to survive the trip. And since time is meaningless in the QZ, they probably wanted to keep the casting options wide open.