It's your responsibility to let them know you do NOT want to renew your insurance after expiry. If you don't indicate your wish, they'll automatically renew your insurance. However, I do believe that you should have the freedom to switch to another insurance company. You can check with another company to see what you can do.
No. Renewal normally is for another year. you can choose to pay your premium month by month, and when you sell your car before going back to China, you inform them the car is sold and request termination of insurance. You can also pay all the premium of the year right now, which is cheaper than that by month (or the same for some insurance companies) and request refund when you terminate your insurance.
By the way, Goulx is a broker, allianz is the insurance company. Gpoulx can also sell another cheaper company to you. that's what the broker does their business. Good Luck!