4) The "Plumage will change in 6 hours" message appears always on first feed and indicates the start of the timer after which the colour number value change from feed becomes permanent.
The "Chocobo is starting to grow new feathers!" message is indication that you just crossed "ranges" into a new colour. All fruits proc but you need to sculpt and push to a different zone using different quantity and order of feed.
5) I found it personally much easier to feed one type of fruit at a time and proceed based on the results, since it's easier to tell the general direction the values will take with only one altering factor.

I'd like to know if anyone could corroborate these assumptions since I've found they worked for me the best.