Yeah, I missed my outbound Air Canada flight at LHR after my coach broke down and when I asked about getting a ticket for a later flight, the rep told me that they "weren't supposed to" sell returns if you weren't planning to use both legs even though they were cheaper than a single but she sold me one anyway, just told me to make-up a return leg and then just don't turn up!

Pretty bad really, I guess an example of where the theory of balancing the numbers doesn't match up with the reality that is obscene that they can gouge you for less service, I think it should ultimately be "admin overhead" + "flight(s)". Most people accept that a single can't always be half of a return but it should always be noticeably cheaper.

Yes, airlines will typically cancel the whole trip if you just don't show up for the outbound leg.

I have, on rare occasions, not shown up for the return flight because just buying a new one-way ticket for a new date/time was cheaper than rescheduling the existing return leg. I don't think you're actually supposed to do that either.