Q19,Q13,Q24 short seller position curicum V4 fixed income P1 understand, increase, position, overnight, income

willsucceed

Q19, it says short seller position can affect the demand of collateral, I could not understand, is a lender or short seller?
how to affect collateral?
Q13, if we use 2 year repo, the duration 2 is greater than overnight, since leverage increase the duration, why the answer says shorten the total duration of portfolio?
Q24, contigent immu. should  use intial portfolio for passive and safety margin for active, why answer says total portfolio use for active?