I'm a big fan of 6A. I'm a little biased, living right by Dundas West, but I think there are some good reasons it should be seriously considered. Although at least in 6B the DRL could be extended westward.
Queen/Roncesvalles station could be a major node. The 501 Queen could terminate there, and a new upgraded higher-speed streetcar could operate reliably west of there (507?). From Humber loop, a ROW would just have to be created west on Lakeshore, and maybe even a branch on the Queensway. This would provide very attractive service to downtown with only one transfer. The Roncesvalles streetcar yard could be transformed into a major junction station, seeing as the TTC already owns the land, and the yard has to be retrofitted anyway for the new streetcars, so why not just find another more suitable property for the new LRVs.
You could still have the Dundas West/Bloor, Queen/Dufferin and/or Exibition stations, and either make them terminals, or charge a high premium during rush hour to continue on to Union station. This would entice many passengers to transfer to the DRL (which would be much more convenient), and bring in more cash for the system. As for platform space at Union, going to EMUs could solve much of the problem I'd imagine, as well as better scheduling service. I don't have too much knowledge on this, but I'm sure there are stations in major European cities that see more passenger throughput on less trackage.
I don't think the DRL necessarily needs to be under Queen, although I don't have a huge preference on this. I just hope it wouldn't mean removing a streetcar line. The streetcar lines would be just as important if not more used to meet local demands generated by the DRL.