The Farplane is where dead people go if they are:

A) sent by a Summoner
B) willingly accept their deaths

The FMV of the Farplane is probably just to reinforce the feeling that just like Spira, there's something a little off about it. That being said, it's more or less an artwork of how pretty the Farplane is. It's a big place, and the color palette isn't exactly a collection of cool colors like the rest of Spira is. Most likely, it's a section of the Farplane that you don't ever get to interact with.

In FFX-2, you get to play around a little longer in the Farplane, but you never go through the whole thing.

It reminds me of the last place where LeBlanc and her goons are, selling stuff, of where the cutscene where Yuna meets Shuyin, and when she makes the decision to see or not to see Tidus again.

We know from FFX-2, that it has multiple entrances, no less that they are at resting places of the Fayth. It's not unfeasible that if you dug around Spira long enough, you might be able to get there, but you would have to have some form of magical assistance. In FFX, the Guado are (racially) the keepers of the Farplane; evidence of this is when Seymour asks Auron in Guadosalam "why are you still here?" Big giant red lights should be flashing, telling you "lol Auron's dead." That explains the entrance in Guadosalam.