It isn't really a 6 year contract as the concept is illegal here in HK. You don't renew anything and you remain on the T&Cs that you sign. Yes throughout your career, you will not get the allowance (but it is being worked on!)
DEFO is the same as FO! Except for the housing allowance.
No. You will get one type rating on either the 343, 77W, 744. I doubt you will get moved about as a SO. Now an SO is not really recognised by airlines outside of Singapore, CX, KLM. Therefore after 4 years as an SO, regardless of how many hours you have (which still might not have reached 1500 to unfreeze the ATPL as you are only a relief crew) , you have not actually done any take-offs or landings! So you actually are pretty worthless at that point, even after 4 years. You get the experience as a JFO for 2 years.
So techincally speaking, this is CX's playing cards. A contract can not be enforced for 6 years, yet it is improbable that any airline will take you during those 6 years.
Again, this is something to keep in mind when making your decision.