First time to Paris..Planning..

That means you will only have 5.5 days in Paris. If you only want to visit those famous tourist spots in the city centre and 1 or 2 places in the rural area. That's enough.
It depends on your choice. In my mind, Louvre is the best museum of the world. La Tour Eiffel is also a place that you cannot ignore. Also, you have to visit Montmartre and Le Scare Coeur.
Actually, all agents offers same things... so choose the agent which can issue the ticket to you at once.
CX and AF offer direct flight and the departure and arrival time is very good. You can maximize the time in Paris. SQ is good in service but the place is a bit old. BA, forget it... For me, CX will be the first choice if the price is only 10% more than SQ. Otherwise, I will take SQ.
In Paris, metro is the most realiable transport partner.
You don't have so much time to go to other cities. Maybe you can consider visiting towns near Paris, e.g. Versailles, Fontainbleau, St. Denis, etc.
Disneyland Paris will keep you for a whole day... Of course it's bigger than the one in Hong Kong. If you like theme park, it will be a good choice also.