1) Use the the "for" loop condition to set ranges in an array. For example with a array of size 100x100 this will set the bottom right quarter of it to one:
For "col" from 50 to 99
For "row" from 50 to 99
--- Array: set at (loopindex("col"), loopindex("row")) to 1
2)Same answer as for 1, just use different ranges with the for loops.
3) There are a few topics on this. Best results come from finding a method and tweaking it till it's satisfactory. If you specify exactly what you want the terrain to look like then we could help you better.