3 (a) That's right. Volenti non fit injuria is commonly raised in situations where the plaintiff has accepted a lift from a drunk driver. What matters is whether the plaintiff was aware of the risk being taken, as in this case. The defence is available to the defendant if the plaintiff knew that the defendant was drunk when the lift was accepted. This is often not the case, for example, when the plaintiff is also drunk - for example see Insurance Commissioner v Joyce (1948) 77 CLR 39 and Joslyn v Berryman; Wentworth Shire Council v Berryman [2003] HCA 34.
Note that under relevant legislation this defence is not available in New South Wales or South Australia against claims arising from motor accidents. The damages claimed by the plaintiff are reduced instead.