Why did Itachi seek Tobi's help?
The things he told Sasuke during their battle suggests that he thought that Tobi was Madara.
Either because eliminating the entire clan on his own was too tough a mission for him, or because he wanted to infiltrate the Akatsuki to gather information. Also, he had Tobi promise to not hurt the Hidden Leaf or Sasuke.
Because that would blow his cover. He was still protecting Sasuke and the Leaf and revealing his position would make him lose all the things he could have gotten due to #2.
Because, as Madara, Tobi too had a grudge against the Uchiha. Also, he could use someone like Itachi in Akatsuki.