Yes

Yeah, my wife got a similar plan with Unicom, although she canceled it before she left because it just wasn't worth maintaining it. She also saw that I was using my China Mobile HK SIM (uses China Mobile in the mainland but doesn't require a VPN) and had coverage in parking garages and elevators where she had no signal. So I think she might try China Mobile next time, which was another reason she canceled it.

No. Prepaid = pay prior to use, and if you run out of your allotment (i.e. use more data than you've paid for), then you either get throttled or cut off, until you have added more data to the account. Postpaid = pay after use, generally requires a credit check, and if you go over your allotment, you get a bigger bill after the current cycle ends. The presence of a monthly fee isn't relevant to whether an account is prepaid or postpaid.

Prepaid is usually an advantage if you're using something temporarily and don't want to keep it long term. For example, I needed a second SIM card for when I was streaming the Disney parks in Anaheim so I ordered an AT&T prepaid SIM. I could not do so much as even make a call or send a text until I paid the bill for the first month, and then when the month was up, I was cut off. But that's the advantage of prepaid: if I don't want to keep the plan active, all I have to do is not pay the bill. Had this been a postpaid account, I'd have had to cancel it before the month is up. If I didn't, I'd start getting billed for the next month and even though I'd get a prorated bill based on when I actually called them or ran into a store to cancel, forgetting to cancel means that you're on the hook for more fees, whereas for prepaid, I don't have to actively do anything to terminate the service.