I could see an option using 6th Ave that gives several options for where you're at-grade or underground - most importantly it would be shallow cut+cover through areas that won't suffer much from the disruption:
WB transfer would still go to 1st St SW station (missing on this map). Could do a station at 8 Ave (or leave it for an infill). Grade options:
1. Underground the whole way = 1500 meters; 1-2 stations
2. Come to surface north of 9 Ave (~300m UG), 8th Ave station at-grade then interline for 1 block before turning west on 6th and going UG again at Bow Valley College (~500m at-grade), UG until north of 4th ave with a station at the Bow (~700m UG)
3. Come to surface north of 9 Ave (~300m UG), stay at grade across Macleods for surface station at Bow, then go under 5th and 4th
4. Come to surface north of 9 Ave (~300m UG), stay at grade the rest of the way
I could see arguments for any of the above. I really don't think at-grade crossings with 1-way streets are that bad. Main disruption is sequential signal timing, but who gives a shit? So then you gotta weigh the brief interline with Blue Line east; it wouldn't be hard to schedule in the near term, but could be a problem down the road, but it wouldn't be impossible to do an underpass if necessary.
The Big Head Plaza would actually be more awesome with less vehicle traffic and the occasional train...that actually gives me world-class city vibes moreso than a low ridership subway.
No beltline, no Eau Claire, no PIP impact, no bridge over the Bow River. Shortens the route through DT by about 600 meters overall