I think you have quite a bit of playing around with Aeroplan ahead of you. To get to Entebbe (EBB) on Star Alliance you'll have to go through Cairo, Addis Ababa, Johannesburg, or Istanbul (or maybe Brussels). So at least one stop somewhere from Toronto. To then get to SEZ on Star Alliance you'll have to backtrack to Addis Ababa or Istanbul (there are a few more options but I think you'll then have three flights to SEZ). Then you need to get home from SEZ. That's probably going to get expensive since it won't qualify as a stopover. As far as timing goes, start looking asap. SEZ is tricky!

Just had a quick look for you. TK has availability and RT business will cost you 300,000 to EBB. To get from EBB to SEZ on Ethiopian will cost you 160,000, butcher calendar has not been published that far out yet. So 460,000 total.