I go back and forth on the Yunnan question. For all the reasons you point out, @simon33, I like to hit Yunnan J1 when I can…but I also consider it crucial to clear Yunnan on J2, and I find that if I’m declaring war on J1, I often burn through all of my land units and wind up with nothing to hit Yunnan with on J2 except for the 2 infantry that start in Siam. The Kwangsi infantry go fight and die in Yunnan or Hunan, the Kiangsi infantry are stuck in Hong Kong even if they survive, and my 3 transports are busy with Borneo (1x) and Philippines (2x).
So if China stacks Yunnan with, e.g., 8 infantry and a fighter on China 1, plus 5-6 units from the UK, then even with no bid, I don’t necessarily feel comfortable attacking that stack with only 2 infantry and air power. Sometimes I will, if the J1 attacks went well and the USA seems busy, but if I lost a plane killing the UK BB and the USA is going KJF, I don’t necessarily have 3-4 planes to spare to kill Yunnan on J2.