I don't know why they decided on building this runway there either. That's before my time here. There are plans to build another one (05R/23L) between taxiways H & N extending south west from N5. Why this wasn't done first, I don't know.
Why weren't there that many incursions in the first 10 years? Don't know that either.
In the last 5 years there have been 25+ incursions. The TSB are the ones considering the parallel taxiway in between the south parallels, but as I said earlier, I don't believe it will help at all.
Sinking the highway into a tunnel would have been a massive undertaking. This section of the 401 is 14 lanes wide and is often at a standstill. Imagine the impact on road traffic if such a tunnel were to be constructed.
Departing from the outer while landing on the inner: I have mused on this one in the past. It can be done but the crossing traffic would pass in front of the glide slope antenna. We could promulgate approaches with no glide slope and/or restrict such ops to above certain weather limits. The outer runway is about 600' shorter than the inner which would cause some crews to request a longer runway. This is already often the case, but it would increase.
I too am a bit of a dinosaur (30 years in) and in the 5 years or so I have left I just hope that this worrying trend doesn't result in an accident.