Absolutely you can't purchase duty free upon return to Toronto.
I usually have connecting flights from the U.S. and I would either purchase duty free at the last connection (Washington for you), or I purchase at a liquor store before I leave (Orlando for you) and put it in my checked luggage. I haven't had a broken bottle in my checked luggage yet, but I still worry a little.
I don't usually face multiple secure areas (except $#%&@ Heathrow!). If you do face multiple secure areas, ask the duty free shop before you purchase. They might tell you to go to your departure secure area and use another duty free shop, or they might be able to deliver it anyway. Either way, they'll know because you can't carry your duty free purchase around, they MUST deliver it to you at the gate (they need to look at your boarding pass for that). They should know when they look at your boarding pass if there would be a problem, but sometimes people are in a hurry and get careless, so I'd ask them specifically.