1. One way does exactly what it says on the tin. However, this does not mean it will cost half the price of a return ticket on long haul, and may even cost more considerably more than a round trip! Plus you would then have to buy the one way return trip.

2. With an open return ticket no return flight has been booked, and you would have to arrange to return during a set period (say 6 months or a year) on a date when there is the appropriate availability in the right booking class. May not be available on all airlines/routes. Will probably include an admin fee for arranging return trip and are likely to cost more than a standard return. More common with various specialist, student or RTW fares and you are unlikely to see it as an option booking yourself on an airline's web site.

3. A fully flexible return ticket would allow you to change your ticket's return date, usually without a fee. However the ticket will cost a lot more than a regular non-flexible ticket. A cheaper ticket might allow changes for a fee, and the cheapest will not allow any changes at all.

Cheapest option is to pick a return date and stick to it! I'd avoid buying single tickets, as there are almost certainly not going to be a budget option. You'll have to ask around with travel agents to find out about open returns - they might be a good deal to China, but could also end up very expensive. You can try making a few mock bookings online yourself to find out what sort of flexible ticketing is available - you might be able to find a reasonably priced ticket where the cost of changing the return date is not too high and so works out cheaper overall than an open return.

At a guess, I'd say you might be able to get a good deal with a specialist TA on an open return, but it is worth checking carefully the rules and fees for changing a regular ticket that you might be able to pick up yourself. Prepare for a spot of research!!