To answer your questions:
1. I think it is hot to get there in Auguest. The temperature will be over 35 degrees at daytime. Not very good in terms of climatic condition.
2. You can have two days in expo, two or three days in getting around Shanghai, then have a short trip to Hangzhou, Suzhou or nanjing which can be reached by bullet train from Shanghai.
3. discounts in cost of expo tickets? Aged people above 60 years old, disabled people. children (free for kids less than 120cm high) can get discounts. It seems there is discounts if booking before June 30?! I am not very sure about this.
4. Take some food while visiting expo for food in the park is very expensive. Drinks' price are ok. Do not waste time on queue, for there are many places to see in the expo. Stay cool and have fun!