In Mahayana Buddhism, do they expect everyone to become a Buddha? If so how can any Buddha help anyone to attain enlightenment? In Mahayana Buddhism, do they expect everyone to become a Buddha? If so, how can any Buddha help anyone attain enlightenment? Isn't the whole point of becoming a Buddha to help others to attain enlightenment?

It's like asking everyone in the country to become kings. Then there won't be any countrymen left to rule.

To give another analogy, imagine patients refusing to follow doctor advice saying that they want to become doctors first to help cure others. Then those future patients they are hoping to help would also have to refuse their help to try to become doctors themselves. At the end of the day, no patient makes use of any doctor and no doctor cures any patient. So why become a doctor in the first place?:)
In Mahayana Buddhism, they assume that everyone is already a Buddha. Enlightenment (bodhi) is self-existing, it is our natural state. It is our true nature, our primordial face. The only problem is, we got alienated from it, lost connection with it - but only at the superficial level. Deep inside we are always enlightened, all we need is to connect the layers and remember.

So when someone attains enlightenment, they see everyone as enlightened already. They also see the confusion, or the illusion of confusion. And that's what they help the others to overcome.

It's like, you wanted to become free from slavery and you thought the only way to do that is to become a king. But then you realize that this sort of thinking is still a slavery thinking. You realize that in fact we're all already free, that slavery was a lie that we believed in only because of ignorance. But as you go around on your white horse helping others get this, some may mistake you for a king. And then it's up to you to either tell them the truth, or to play a king for their benefit, like an adult playing Santa for the sake of kids.