1. "You were responsible for this" . Is this correct.
2. "He
has many good qualities
. I would also like to cultivate few of them". Is it correct to use 'Cultivate' here?
You can, though it's not clear if you want to cultivate them in yourself or develop them in him.
3. Can you please explain the use of "In leiu of".
In lieu of = instead of / in the place of. "take that DVD player in lieu of repayment for the TV that went wrong." Note spelling.