Essentially, ignore the two letter codes and read the three letter codes only.

Treat "/" as meaning that the items are alternatives. "-" means they are sequential, but optional.

So, taking number 2: Rewrite it as:
YZF to YVR to YYC or YWG or YEA to YTO or YOW or YMQ to (any of the eastern seaboard cities) to ATH to MIL.

As you can skip items there, that means that YZF-YYC-YYZ-ATH-MXP is a valid route. And you could also add in a connection in YVR before Calgary and in, say, New York before crossing the Atlantic. You could also leave out Athens if you can find a direct flight to Milan.