For international travel, the rule basically amounts to stay with in the MPM. We did troubleshoot a situation recently where someone was trying to get to somewhere in Scandinavia and DL had an MPM that was absurdly close to the great circle mileage (like within 50 miles or something equally crazy), meaning that routing was only legal via one DL hub (DTW, iirc). KL publishes routings listing where connections (formally called transfers in the routing/fare rules) can occur, so I encouraged that poster to try to persuade the DL agent to use the KL routing rules on an exceptional basis, since DTW-AMS (or vv) wasn't available on his required date but plenty of other US-AMS flights were.

As to thepointsguy's BCN-CDG-YUL-JFK, how it's legal is very simple. The mileage is 4301 and the MPM is 4592. Doesn't matter that the routing doesn't make perfect sense.

Finally, relative to the question of whether transfer locations make a difference in resetting the MPM, the answer is no and I think that's pretty unequivocal. You've got an origin and destination, and the transfer points create the segments that contribute to the cumulative miles. About the only way you can make clever use of a transfer point is to basically make it the destination if that buys you more MPM. Of course, with only one stopover allowed, you'll have issues leveraging such an event, but maybe an open jaw could come into play. For domestic awards, it seems that routing rules are (usually) off the table, as long as there exists a published routing, you can probably book something that's not really indirect even if it's not the officially sanctioned routing. This winter we had the unfortunate situation of someone who wanted to fly between two cities for which DL had destroyed all valid routings by making one of those cities served only from ATL, and the only published routing was via MEM. Thus, the award was pricing out as two round-trips, both with ATL as one of the end points. (Revenue tickets were also pricing that way because of the stupid routing rule.)