Ms goodness! Please tell me again:
Why mutlular instead of mutlu?
Why not mutludur?
"mutlular" isn't required, but is a totally normal thing. If the subject is marked plural, the verb doesn't have to be. Using the -DIr suffix is for strong assumptions or encyclopedic facts. This is neither of those :)
why not "Çinli kisiler çok mutlu" ? "Kisiler = people,
It already has the concept of "people" built into it.
I wrote Çinli insanlar çok mutlular and it was accepted, so kisiler should be also, no?