1) what % highway dirt is this ride
My best guess is 50% highway (400 miles highway, 4 days of offroad at 100 miles / day). But the highway is all done at very low speed, like 55 MPH cruise. I led a group last year on a 65 mph CRF230F with no power in reserves, haha.
Assuming you trailer to McAllen (Mission to be exact), and ride with the big group, it is about 160 miles of concrete to the beginning of the dirt road in Rayones heading to Galeana. Along the way you'll cross the bridge at Anzualdas, stop for a snack in China, eat a nice lunch and gas up in Montemorelos, and then begin the dirt riding in Rayones to Galeana (or longer if you choose). You can map this on Google Maps, just remember the towns are in the state of Nuevo Leon, Mexico. So google Galeana, Nuevo Leon, Mexico as an example.
Most days expect to ride 5 - 20 miles of concrete to and from the "trailheads" for the rides listed in Richard's excellent guidebook. And then between 50 and 150 miles of gravel / dirt each day. The choice is yours where and how and with whom to ride.
There are a couple of all concrete or mostly concrete rides that are fabulous and take you to obscure and neat destinations like a giant waterfall.
On the return trip many choose to do concrete all the way from Galeana back to the border. This is about 201 miles back to the hotel in Mission.
But a very cool option on the way there or back is to ride north towards Monterrey and ride some exceptional dirt and concrete roads through deep canyons. A smaller group of faster / experienced / mexican savvy adventurous riders usually does this either there or back. This might bring the concrete portion to 160 plus or minus.
So minimum, you could keep concrete to 320 miles or so, and ride dirt another 4 - 600 miles. So best case 30% concrete. In any case, pure knobbies meant for hard rocky conditions will be fine. And in the case you are more street oriented in tire choice, I've done many of the roads on my 80 street /20 dirt (per the manufacturer) Shinko 705 and the rocky conditions are fine on this mainly street tire.