Even with a Front street allignment, those locations are too close to Union to warrant a seperate station. Exhibtion place would equally be served with a Queen street alignment.
You're shooting your own argument in the foot by applying such random walking distances to the various alignments. If a Queen subway can serve the Ex, which is south of Front, then a Front subway can serve Queen at least as well (much better, even, since there's more condos and jobs and clubs south of Queen).
A lot depends on how far we think people walk after taking the DRL. If people will walk up to 750m to their final destination, a thru-Union alignment serves everything between Queen and the Lake and, likewise, a Queen alignment serves everything down to Front. If people will walk only 250m, though, a thru-Union alignment is less useful for Queen or Queen's Quay (another reason why both a DRL through Union and a separate Queen line might be desirable). But if people are only going to walk like two blocks, a Queen alignment is
not a bit better for "relief" since people would transfer to the DRL and transfer again to the YUS loop to go south to all the office towers. Of course, they won't transfer twice, they'll just stay on the B/D line and transfer once...
There is no alignment the DRL can take that would help every YUS loop rider...which is fine because the point is to offer a good alternative to some, not all of them. We need to look beyond the YUS loop to really compare the utility of each alignment, but this can't be done in a sensible way if people dismiss everything at and south of Front as the edge of the earth and an uninhabited wasteland, which it clearly is not. There comes a point when tower after tower after tower in CityPlace or the Portlands starts generating more trips than a low-rise commercial strip that may not even still be trendy by the time the line is built, lined with neighbourhoods the city will not permit much additional development in.
The rules of the game would change quite substantially were the DRL to run up Don Mills, since massive numbers of people would take the DRL straight downtown and not travel to the Yonge or Danforth lines.