OK, I think I got the idea. But I still have some questions!
(1) It almost seems to work. But the graphic tile isn't shown at the main screen. I at first thought it's because I haven't added the mysterious 'BLANKS': it wasn't on the availabe map files. But everything worked fine. For curiosity's sake, I added the word 'BLANKS' in the MapEdit.dat, but then it just crashed. So is it natural for the graphic to be not shown? If I click the Avenger or Lightning, the overal hull can be shown on the main screen, unlike the Miner.
MapView automatically adds BLANKS, so you don't need to add it to the tilesets list.
(2) So there are Top, Tile, Rmp view, and it's impossible to see all of them at one screen at the same time. Is there any way to see them all at once? (I'm wondering if there is a way but it's just me not being able to find how to do it).
Only if you change the window sizes so that all will fit the screen.
(3) Here's the picture of the RmpView. My mission is to get rid of the nodes where the enemies spawn: I've seen them spawning on the ground floor and on the 1.5 floor or something (they spawned on a stair up to the second floor, I guess).
But before removing anyting, I'm just getting more and more curious. So the 'type' seems to be a factor deciding which can pass the point and which cannot. Then what does the 'rank' do? Does it decide which units to be spawned? But I've never seen a civilian spawning inside the craft, unlike the aliens, and the rank is set to '0: civilian' (or civ-scout, whatever it means). Lastly, there's also that 'any' part on the 'use info' section. So I think it's a factor deciding which can pass through the line between the nodes. Then... which factor decides which unit to be spawned? Maybe those numbers in the 'spawn' but then I have no clue which refers to which 
Thanks as always 
Type determines the type of the alien spawned on that node
Rank determines the unit that will spawn there: 0 = Civilian/terrorist/scout (aliens set with a flag to start outside UFOs), 1 = XCOM, the others are for aliens and their ranks
Use Info sets the type of alien that can use that connection
Spawn is the probability that a unit will be generated on that node (0% - 100%)
