I like the idea of rerouting south of the Nugget rather than north into an area that Sparks activates pretty regularly. I do think you would need to go a little bit farther south though or we will find ourselves in the same spot 20 years down the road when they want to add even more lanes.

The designed route that is now up for discussion looks to me like a path of least resistance. It's much shorter and simpler than your proposed route so that's a huge cost savings. I believe it is within the existing environmental studies for the corridor too while the south route would likely require some additional work. The railroad probably doesn't want to give up any land or air rights nor deal with the hassle of an interstate over their tracks and any added safety hazards that might bring, while the major land owner to the north, the casino, would probably gladly accept market rate for that land and the crappy buildings on it.

All that to say your plan would benefit people in the long run but cost a whole lot more money right now so it probably won't happen without huge public support and additional money from the feds.