Rerouting the Southwest Line through the Midtown Greenway to join up with the Hiawatha line is not the answer to the current LRT debates. To start, you really need to examine the existing route. It doesn't take much to understand the purpose of this line is to get suburbanites out of their cars and onto a reasonably rapid form of rail into downtown.
Once you get past the West Lake station, the remaining stops are minor in comparison to those further out along the route, and the fact you don't end up on street level in downtown makes for a quick last leg of the commute. That seems remarkably similar to the original concept of 35W as it heads south from downtown. There was a reason they didn't any ramps until 35th Street: they wanted the people who lived within 3.5 miles of downtown to use other means to get there so the freeway wouldn't be so crowded.
All that aside, let's consider what it would look like if we were to send the Southwest Line through the Midtown Greenway. It's hard to picture this without stops at Hennepin and Lyndale because of the dense population in those areas. You can easily make an argument for Chicago because of the hospital and medical infrastructure. It goes without saying that you'd need to include Nicollet because of the future trolley line, and what about Cedar or Bloomington as the last stop before merging with the Hiawatha Llne?
Are you catching my drift here? Aim the Southwest LRT down the Midtown Greenway and by proxy you'll add as many as 5 stops to the line, plus the existing ones on the Hiawatha line from Franklin Avenue into downtown. On top of that, the stops are close together and support a dense population. Add those factors together and you've just increased that suburban commute by a considerable amount of time. On the other hand, if the Southwest LRT departs the West Lake station to head downtown through the Kenilworth corridor, there's only 3 lower key stops before you hit the downtown transit hub. If we slow that commute down, the byproduct would be a drop in ridership outside of Minneapolis and that would doom the ultimate purpose of the line.
Get onto Google maps and check out Melbourne Australia if you have any questions about this. Melbourne is generally considered to have the best light rail transportation in the world and I've seen parts of it firsthand (both as a rider and driving alongside smaller rail on narrow city streets). Their transit plan is based on the concept of three forms of rail transport, along with a conventional bus network.
That first form of light rail transit is dedicated to bringing commuters in from the suburbs. To accommodate this, they re-purposed the old downtown rail station as their metropolitan transit hub and kept all of the original incoming track. Would that we had the foresight to have done this in Minneapolis, but the old train station has a less significant purpose now and the track leading there is long gone. At least St. Paul kept their infrastructure and the day will come when they'll be able to showcase that beautiful old rail station as the east metro hub. Once a commuter disembarks at Victoria Station, they can jump on street level rail to get further into downtown, or the free trolley that runs around the periphery of the central business district. One critical point to note is the rapid transit rail is located in trenches below street level, much like the Midtown Greenway, until it gets a certain distance from downtown. The trench guarantees no interference from street level rail or automotive traffic. Finally, the rapid transit stops are rarely less than a mile apart, and each stop supports platforms around 400 ft long. That's right: they chain together as many as 6 cars for that rapid rail to the suburbs. The 3 car max we have on the Hiawatha line is small in comparison.
The second form of light rail transit is similar to what we'll have with the new University line. They radiate out from downtown with some interference from cross traffic maybe every 4-6 blocks, but they don't share lanes with autos. Another item is that you'll see only single rail cars running on these lines. The reason comes when they get around 3-4 miles out and they split off onto smaller streets. It's here where we see the final form of their light rail where the single cars share lanes with autos, much like what we expect to see with the future Nicollet trolley line.