I replayed it from the Falrog. It worked this time, it seems that it's necessary to be wearing the Breastplate when fighting the bugs so that it works.
No, you don't need to be wearing the Silver Armor, I just did that recently and I was wearing my Dragonheart breastplate the entire time. I still got the silver armour.
1 more question: Could it be that Cure quest for the 3 plague victims has not right solution except for pickpocketing 1 of the victims after giving them the potion. i did that and it worked, but why wouldn't they make u choose 2 out of 3 ppl?
The quest is meant to force you to choose one person you can't save. That's the entire point. Unfortunately, it's sabotaged by the surprising lack of characterization for the victims. A lot of relatively minor people have interesting backstories, paragraphs long, and yet when you're faced with a choice of letting someone die to save the others, you don't get much at all on what the people are like.
I also don't like that you can solve that quest by pickpocketing a cure, because that would not work in reality. And if Larian wanted you to cure all three, they'd have put 3 cures in Elrath's cellar, not two.
That would be one case where it would have been better to delete the cure items after they're used, instead of moving them into inventory.
