The SE LRT started as a two prong spur line from the south LRT as you propose, but from Anderson. When the planning got further along, it was realized that the south LRT didn't have enough spare capacity to serve the service area, so a route needed to be laid out to connect the service area into downtown. CN was more willing to part with ROW since it isn't the main line compare to CP.
Here were the considered routes in the 2003 study:
The other questions:
I'd be very concerned if the buses were anywhere close to full in Mahogany.
One of the reasons to go LRT with a tunnel first is to get travel time savings, and travel time savings equal ridership.
The ridership estimate for the West LRT was bang on (within the confidence intervals). The city has a pretty good model.
Why would Ogden not TOD? In West Calgary the organic 25th Street strip has turned into a way better TOD than the attempt at a master planned TOD at westbrook.
There isn't a route to the deep SE that doesn't go past the dump. Thank Peter Lougheed for that.
In the end, the questions you ask are: can't we build this for the city we wished we built over the past 70 years, instead of the one we built. The deep SE was really cheap for developers to build on, but it is really expensive for the city to service. If we were to turn the clock back 40 years, we probably would have tried something entirely different, because the city sorta shifted on the fly after it was told Bow Bottom Trail couldn't cross Fish Creek except for the old old old bridge to get to Sikome Lake.