This is super helpful!!! Thank you!!Your ranking is correct per square, but one thing to consider is that mana buildings require roads. Because of that, the Weeping Willow is my top pick for mana producers from the build menu in Ch. 9. You would need so many more road tiles to get the same amount of mana from the tiny Treant's Grave, making it much less efficient in the long run.
Yes, it can absolutely be worth it when you first get into the chapter and need to start making mana. I started with 2 or 3 Willows myself, and still had 1 by the end, just for the culture it provided. The key is to be on the look out for other sources of mana in the MA, events (the Magical Nutcracker, today's daily, is the only building in this event that gives mana in your chapter), and seasons. Any of those buildings are almost certainly going to be better than anything from the build menu.
Festival Merchants are craftable in the MA, as are Mana Sawmills. Festival Merchants are the better producer of the two. I personally would not spend precious RR's on any building that can be crafted. It can take a little while, but crafting new ones when you change chapter allows you to use your RR's on more valuable buildings, such as evolving buildings or sets.