2. Palani to Kodaikanal by bus - Yes possible and easy during day time.
3. Kodaikanal to Chennai (Visit Thiruvannamalai) - You will need a plan to stay at Thiruvannamalai for the night. Kodairoad to villupuram by train (preferrably Guruvayur express), and another train from villupuram (Daily passenger starting at 7pm.) Stay for the night and visit the temple the next day early morning and you can reach Chennai for lunch.