Ian, I don't know where you get the magic numbers from...
Just for the sake of comparison (with our usual benchmark, YYC), we'd need around half of this, spread between Oliver and Downtown, to be at about the same population number as Calgary's downtown area (Downtown, Beltline, Eaux Claire, Chinatown and Sunalta). Between these two neighborhoods, we have roughly 30k, whereas Calgary has roughly 42k on the 5 I listed.
As I like to point out, as well intentioned as you might be, you seem to have a little bit of a misconception about the role played by population density on the vibrancy. It has, of course, a relatively big impact, but a change in the urban culture is much more important, and the steps the city planners take to incentivize such change are crucial. Were density and total population the sole (or main) drivers, Halifax's downtown wouldn't be so much nicer and more vibrant than ours.
What hurts Edmonton Downtown the most, in this case, is the mix between the complete lack of good planning, good care and attractiveness to spend time there, even for those who live here.
We complain that everything goes to the WEM or SEC, but these places offer a much more pleasant atmosphere to shop and dine than Downtown, to the point where Downtown residents prefer going to the same stores and restaurants on the West Ed that they have close to home.
I've had a friend who lives in Vancouver Island visit the other day and we took a drive around downtown (it was raining as hell...) And he pointed one thing that we all know: our downtown is not necessarily as ugly and uninviting as some make it seem, when they talk, but it lacks the care. Our sidewalks need to be cleaner and repaired, the pavement on the streets is worthy of a mid-sized third world country, there's too much graffiti and you can walk (or drive) the entire span of the area and we don't have a single beat cop to ask for help or information.
Notice how all of these are things that could easily be fixed and would help A LOT in making the area much more pleasant and inviting. Broader, nicer sidewalks, maybe a completely closed off street lined with outdoor patios and a space for live music (RHW, anyone?).
Even the parking lots, we could have the owners fence it, add some small landscaping around the lots, so that at street level we don't have to see all of the disrepair.
Come back with beat cops patrolling the area, so people can feel safer, give them free range of movement by transit with free LRT within the confines of the area, and BAM! You'll have a downtown where people will actually want to stay, both businesses and people. Won't happen overnight, but give it a year or two, keeping the area like this, and you'll have a resurgence.
Might sound silly, but it's essentially what EVERY city that created a vibrant downtown did, in North America: make it look nice and inviting, make it easy and safe for people to come and spend time there.
Density, and most importantly, vibrancy will follow, not the other way around. Again, there's too much of a case to be made, in Edmonton, that the city is in much more need of a supply shock than a demand one. The nice thing is that by the end of my Master's I should have somewhat of a nice answer to that, with evidence and using solid science. Thanks to you, Ian, I decided to take upon myself to write my dissertation on this