Hi
To the best of my knowledge, they can't cut down what you are entitled to. Once you cross the border successfully, you can stay for 6 months no questions asked.
Perhaps I am wrong, and that they can limit it. Best to phone CBSA (customs) and confirm with them.
Your wrong, once you leave Canada your visitor status ends. You have to reapply for admission, and if CBSA limited your stay, then it is quite likely that if they admit, you will only get to the 15th of August.
PMM