I'm now in my last semester of my 4 years in
NTU Mech Engine.
I totally experienced what you are experiencing now when I was your age. Lower than expected grades, undecided, just want to squeeze into a local uni.
My advice for you is that if you are hardly interested in engineering, don't ever come in.
The amount of maths and engineering science i.e. Fluid Dynamics is enough to kill you. Don't EVER let your mind get into this lame notion that you want a "challenging" degree. Its not worth it and the knowledge is too scientific to be practical (ironic, I know). You're better off challenging yourself running up a hill or something.
I've known people in my course who are suffering in agony. In the end they simply just wanna get this **** over and done with, plus, they are totally lost over their future now. What for?
Pertaining to your enquiry on future job search, I'm undergoing it now and as a damn normal 2nd lower, I'm stuck with applying for engineering-related jobs. 2nd Upper is actually the minimum to be able to get out of this **** (in my opinion). The rest just continue to roll in it.
You really need to be determined to score well in Engineering and 4 years is not a short time. Even if you are determined, you might ask yourself at the end of the day the same "What for?" when you achieved it, because people who are smart enough know there's no money to be made being an engineer. It's all about the passion (I won't ever understand this).
I totally agree with some of the things said here. Go study a degree that's easy to get a 1st class or take a degree with a specialized skillset i.e. Accountancy. After all, this world is all about profits, growth and numbers. It's either you help make money, save money or count money. But in my humble opinion, don't re-take your A's, why so hardcore? You can consider SIM, it's really not much of a difference. The grades, leadership and industrial experience when you are in school matters the most.
I hope this helps. Don't wanna see another guy die a slow death.