Not sure just how many things are included in your list of ´highlights´, but in my opinion two weeks should be OK. Of course there are going to be people that say two weeks isn´t enough, but we all dont have unlimited time and $ do we!
I travelled from Flores (Central Guatemala) to the ruins at Tikal, into Belize, spent some time on Cay Caulker, then travelled north to Tulum. Then up to Isla Mujeres (off Cancun). Essentially I didn´t spend time in alot of places on the Yucatan itself, but as a guide for timing, this could have easily been done in two weeks.
I didn´t see any other parts of Belize (because I didn´t really have the time or $), but Cay Caulker is a must. I did a pretty cool tour with the Rasta guy that parks his old wooden boat at the split too. Nice and chilled. If you are travelling with a few people, try to rent a house (even if it is only for a day or three). We ended up paying less per person than for a bed in the dorm at the hostel.
Isla Mujeres was also great. Away from the wanky scene of Cancun (but still a bit overdone). We hired a golf cart between four people to explore the island for the day. Good hostel there too (its in the LP). You can easily get to and from the island from Cancun. Boats leave every half hour or so, so its easy to get out and back in a couple of days.
There are easy bus connections between Belize and Tulum and Tulum and Cancun.
My tip: make sure you dive the cenotes just outside of Tulum. Even if you are not a diver, snorkeling is just as good. We organised an early morning (8am) bike/snorkel tour through the Weary Traveller (i think?) Hostel in Tulum. Amazing. Make sure you go in the morning if you do cause they say the water clarity can decrease a little if lots of tour groups/divers are in the water. In addition, a day at the ruins and the beach is all you´ll need.
Good luck, I´m sure you´ll have a great time.