1) What I think I've understood is this: you can't place a tunnel tile from a vein unless it's continuing a tunnel that went TO the vein. Therefore at the start of the game, you can't place a tile next to the center starting vein. You're going to have to reach it by some other method. Right?
2) What I also think I understand is that you can't place a tile next to a city space unless a) you're continuing a tunnel TO that city space, or b) you have a station on that city space. Right?
Almost correct. This is true if the tile's veins go into the city. You may put something next to a city space if it goes across without going into the city, but of course, that vein must be extending something as well.
3) You can't place a station on a city space unless it's connecting to a station you already have in the mountain. Right?
Almost correct. It could also be connecting to a station in another city.
4) Therefore, at the start of the game, you can't place tiles next to the center vein OR next to a city. Right?
5) WHICH MEANS . . . I think I get this now . . . the four 4-exit tiles are placed on the board at the start because without them you'd never be able to legally place a tile. Therefore at the start of the game, the first tiles that are placed MUST NECESSARILY be placed next to these start tiles. RIGHT?!?!