If you check out the official site, they have a spectator tool. I think the most you can hit using Disney transit is 4 spots. If you use your own car, you may be able to do more. A few years ago someone did that, and I recall seeing them a few times on the side of road, no idea how well they knew the roads to do that though.
Pretty much the start, then take the monorail to TTC, cheer there, then walk to Grand Floridian, cheer there, then monorail back to Finish is the best they will usually show if you use their transit.
MK and AK are ok to watch from. AK you should still be able to make it finish before the runners do. However DHS and Epcot would be pretty much impossible to do if you want to see the finish. The Beach Club/Yacht Club is a nice place to cheer from, but you won't be able to get to the finish before the runners do.
Anyone can watch at the finish area. To see the actual finish line though you would most likely need to buy a spectator package. That will let them in the grandstands at the finish line. Otherwise you can crowd the barriers about .1 mile from the finish as the runners turn the corner.
Similar stuff applies for the half, but you really only have the Start, the TTC/MK, and the finish to cheer at. If they are going to rely on Disney transit, their best bet would be to get to the start using the racer buses. The rest of transit will pretty much be shut down or heavily rerouted at start time. As such, there is a good chance they wouldn't be able to make it to the TTC unless staying on the monorail loop.
The text messaging system is hit or miss. The first year I used it, they had my 5 and 10 mile splits, but that was it. Never sent the others. They finally got the finish split out a few hours later. Last year, it seemed to be much better, and pretty close to time. However, there can be a delay in receiving them, so its a good idea to have a rough idea of when the racer should be at a particular place just in case the text doesn't come through.