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.
In my experience, travel between Europe and the US is particularly random. Sometimes, open jaw flights are perfectly fine in terms of pricing. Other times, I've had to jump through hoops of one sort or another to do a round-trip from a single city because one-way flights were so expensive relative to the round trip.
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.
TIL, thanks!
On a related note, I found out the hard way that (some?) airlines don’t let you use a return flight if you missed the outbound leg.