China tour - itinerary

This is my suggested route for you:
Apr. 12 and 13, you can just visit attractions in HK;
Apr. 14, going to Macau in the early morning, and spend one day there. And then in the evening go through the Gongbei Border, and take a train to Guangzhou.
Apr. 15 and 16: spend 2 days in Guangzhou.
Apr. 17 and 18: take the evening train in Apr. 16 or the early morning train in Apr. 17 to SZ, and stay at the city for 2 days. At the night of Apr. 17, take flight to Shanghai.
Apr. 19 and 20: stay at Shanghai.
Apr. 21: go back to Hong Kong to take your flight.

For travel agency, i suggested you to contact TravelChinaGuide. It’s really a reliable one. The email address is: traveltravelchinaguide.com. Just contact them as soon as you can to make sure you can get comfortable service.

For the cheapest air line, it’s really hard to say. The flight ticket always changes. I can only suggest you to pay more attention on it, and maybe you will find a economical one.