My wife and I had booked a vacation to London. In the interim a business matter arose requiring me to be in Eastern Europe at the same time. I adjusted my flights to meet up in London and return with her to Toronto on the same flight.

The client flies business class for overseas and I discovered that routing home through LHR on the same flight we'd already booked was cheaper than usual connection in Frankfurt. The risk was the client who has in the past cancelled trips at the last minute so I kept both bookings.

I checked in on-line from overseas but was a no show in Toronto. Air Canada gave the seat to someone else but coming home I asked for and received boarding passes for both seats so whoever sat in Y would have extra room.

That's always been my take. While personally I guess I don't care much one way or the other I probably would get a bit annoyed if there was a line of passengers trying to use the business/first class bathroom. I can understand, and have seen, exceptions made say when they were serving in coach and there was no way to the rear of the plane but that's different than a waiting line.

DVT can lead to serious health issues, even death. In American Way (AA) Magazine, moving ones legs (especially on a flt. over 3 hrs) is important.
On (most) intl. flts, the economy cabin is more than twice as large as (F/BC). So a pax in F/BC, to get the proper amount of leg movement, would have to walk into Y--less essential for Y to go into F/BC.
On page 122 of American Way, Dec. 2015 issue: http://www.ink-live.com/emagazines/a...mber-2015/#122
they do suggest walking,

On a recent AA flight to CUN they were not stopping people going from coach to business. I didn't go back to coach but I don't think it would be a problem.

There was no security risk obviously. Just a tired FA playing at God.

On a recent trip from HNL to NRT on a UA 747, the FA suggested if I wanted to stretch my legs, why not go downstairs and have a walk around the lower deck a couple of times.

Sensible suggestion that seems to be at odds with the experiences related here.

It's been a few years since I was on the upper deck, but I also had no problems walking around.