Getting back to how the Japanese respond to this Fleet, I still have the view that Pearl Harbour comes first, with a later visit of the Kido Butai for Singapore. However they could add to the already reinforced air units in Indo China. Originally they moved the 22 Naval Air Flotilla with its G3M and G4M bombers there to counter Force Z, something they did admirably.
Now they would move the 23 Naval Air Flotilla with its 72 G4M bombers, and 100+ Zeros to Indo China as well. This is going to be difficult, with airfields stretched to the limit and maintenance accordingly suffering. This would be a significant upgrade of air assets, but with a quickly diminishing return as both fuel and munitions would be exhausted quite quickly.
For naval units covering the Thailand/Malaya invasion force, they only deployed 2 battleships, could they rob another 2 from the Pearl Harbour cover force of 6?
Still deploying submarines in picket lines, and laying some minefields, they would hope for some attrition of British battleships, while they run the invasion force around Indo-China into the gulf of Siam, keeping quite close to the coastline.
Dare the British sortie that far north, with war still undeclared?
I need to go back and look at what Japanese dispositions were at the time to see what they could do. I agree they may decide to let the RN come to them.