How is the 2 screws ended up with a series of broken lens?

I don't see relation to two of this.

Most probably, some other expert on lens have to prepare a set of question for both of you to see who's actually at fault here.

If not, it will be never ending bickering among two of you.

Doesn't mean I'm supportive of him but,

Isn't it common sense if you bought a demo unit, you used it for a year, it is still considered used unit.

Demo unit doesn't mean the unit is damaged.

@Silverfire

Izwan paid you the lens before you ship out the item. So I guess, you have to refund him first before he start shipping out your lens. Well, this is applicable apparently because you have lost the trust of many forumer here, and importantly, izwan himself.

I'm quite sure if you handle this issue appropriately in the past, Izwan would surely return the lens first before you can proceed with the refunds, but I guess it's too late for this option already.

You have to start to learn to compromise about this matter. TS is very patience about this case already, so you have to treat it fair and square.

Just do both of yourself a favor, get this issue down fast. The longer you delay, the harder the case is going to be solved.