The good news is you have an excellent understanding about yourself, so that you know how to make a right decision.
The bad news is you seem to be confused about the entire higher education system, which has confused how you make your choice.
In answering your concerns:
1. You should NEVER consider retaking HKDSE unless it is truly necessary. As you have said, your chance of improvement is not high at all. So why do you have to waste time on something that you are not even certain?
It also sounds like that you know you will end up in HD/AD regardless. It is only a matter of what you are going to study. So why bother retaking HKDSE?
2. Sciences is not a major, but an integrated program. Eventually, you still have to pick a major.
If I were you, I would choose the HD program from HKCC (providing an offer will be received). In general, programs from HKCC is better designed than most sub-degree programs in Hong Kong. At the least, HKCC provides conditional admission to PolyU SPEED to qualified students. At the least, your future is more clear than most people who choose otherwise.
Mechanical Engineering may not be your field, but you can request a transfer once you are enrolled into the program.
I have no comment about the IVE program, except to the fact that since you know that your chance of improving HKDSE is not high, it is a good alternative for you without wasting any additional time.
Remember - you can still retake HKDSE and apply for JUPAS even you are studying in the HD program you mentioned (as they are all self-funding).
2015-07-17 18:12:26 補充:
No offense. I have to go personal this time.
Bid, can you kindly explain why your explanation contradict JUPAS:
http://www.jupas.edu.hk/tc/j4/applications/eligibility/application-via-jupas
JUPAS has clearly indicated private candidates are eligible for JUPAS based on HKDSE results only.
2015-07-17 18:16:14 補充:
Also - how do you come up with 3.6 CGPA?
When graduating with First Class Honour is 3.5 or above:
http://ugadmin.ust.hk/acd_reg/
http://www.ouhk.edu.hk/wcsprd/Satellite?pagename=OUHK/tcSingPage&c=C_REG&cid=191209016800
Assuming you are correct, that means no one can get back into a UGC program?
2015-07-17 18:16:32 補充:
The more I see your answer, the more concern I am.
2015-07-17 18:19:25 補充:
If I were you, I would not believe that Bid has answered (See Comment for detail).
Sorry about being personal.
In response to your concerns:
1. Getting better in HKDSE does not mean your English will improve. In fact, many sub-degree students use IELTS to prove their English level instead.
2015-07-17 18:24:28 補充:
Continued at Comment.
2015-07-17 18:27:05 補充:
Continued:
2. No offense - you are scared of your future, which is understandable. However, if you know you will end up with HD/AD anyway, why wasting another year?
What makes this worse is - you know your chance of improvement is very slim. You still want to repeat anyway.
2015-07-17 18:29:15 補充:
You are in fact avoiding your future.
3. There is a reason I suggest HKCC, as HKCC provides conditional admission to PolyU SPEED for those who have 2.8 CGPA.
It is not your best solution. However, it is a nice backup.
4. You can alwas switch program, as soon as you have good results.