Hummmm...Looks like you got some good advice as your route is pretty complete. There's not much along the coast from Matamoros to Xilitla so I would cross at McAllen and ride some dirt south of Monterrey and pop out in a town called Galeana. I rode here with a buddy of mine in 2010. Here's a link to the ride report...
The rest looks good except you'll not get any better beaches on your trip than in the Yucatan. You might make it to Tulum by New Years. Then go through Belize and into Guatemala through the back door. Hit Tikal ruins and then connect up with the rest of your plan. If you have time...before Tulum...stop in Palenque and stay at the Mayabell campground next to the ruins. Its a really cool crowd and they have live music almost every night...good times!