Haha, well you have a point and I've got no idea what the timeline will actually look like, but in my head it goes something like this:
Barney gets engaged to Quinn per her essential invitation (and if that girl aint a gold digger looking for a better retirement plan...)
We find out in the finale that, surprise, Robin is his future wife and thus we know Quinn won't work out
Quinn sticks around through the beginning of next season, but then the relationship ends early on
Barney and Robin get engaged during November sweeps (or they get together in November sweeps and get engaged during February sweeps)
Barney and Robin get married on next year's season finale
All the while, Ted has been avoiding Robin until the moment the bride requests to see him
Why might she be requesting to see him after all this time? Well, Ted made this promise a while back to be Robin's best man....and she needs him now.
That all being said, it would be incredibly difficult to keep Robin and Ted apart for that long, and it would also be a little sad considering next season just might be the last. I do like what they're doing with Ted and Robin though because it feels much more real to me than I often see in sitcoms. Being around your ex is not always so easy.
Some might say they're addressing Ted and Robin's issues a little too late, but I suppose it's better late than never. While this episode was a bit short on the gut busters, they did have some good jabs, but I found the overall episode more about moving the plot along. It'd be interesting if the show writers showed Barney's engagement to Quinn in the finale, then "BOOM' flash forward, Robin's the bride.