#1. Mostly true, Texas is huge however and there are MANY gaps where there are unaffiliated counties, including the one I grew up in and the ones that border that one. Land percentage wise that's a huge chunk of the state, though granted population wise it's pretty insignificant. Still - room to grow.
#2. In most counties in Texas, with the exception of Harris and Travis county, just showing up and asking will probably get you a seat as a state delegate. To be a national delegate pretty much means showing up as a state delegate and wanting to be a national one. You can become a national one without being a state one, but it's safer to take the state>national route.
I've been a state delegate three times. I've got to back you up on what you've said - showing up is about 90% of the process. I've had friends ask how I pulled off being a state delegate, especially the first time. Answering "I showed up" didn't seem to register with them as much as I had hoped it would.