I agree that you should stick with visiting Tikal, but you should fly down directly from Guate at the very beginning of your trip and only dedicate two days to the site, with one night spent in the park.
Here's how I would edit your itinerary:
Day 1 (Sat, Feb 4): Arrival from Vancouver - in GC around 10:15 AM. Visit national museum near the airport, then fly to Flores on evening flight.
Day 2 (Sun): Morning in Flores, travel to Tikal in PM and buy ticket good for the next day after 3:30pm. Night in park.
Day 3 (Mon): Tikal, back to Flores in afternoon and evening flight back to Guate. Shuttle to Antigua and sleep there.
Day 4 (Tue): Antigua
Day 5 (Weds): Antigua or Lake Atitlan
Day 6 (Thu): Lake Atitlan
Day 7 (Fri): Lake Atitlan
Day 8 (Sat): Lake Atitlan
Day 9 (Sun): Chichicastenango market, return to Antigua in PM. Sleep in Antigua
Day 10 (Mo) Antigua all day. Afternoon shuttle to Guate. Stay in Guatemala city.
Day 11 (Tue Feb 14) Departure - flight back to Vancouver is at 11:15 AM

