1. that's completely free. you can put the objects, the textures and the config in separate pbos or put it all together or include it in the came pbo as the island. that's no problem. imagine ofp handles pbos like your computer handles folders. so only the cross-references must be right. that's all
the most convenient way though is to put the island in one pbo and the objects and their textures etc. in another one because that makes it easier for other people to use your objects for their own island.
2. not as far as i know. (perhaps the old tga2paa programme???) i might be wrong here...
btw. the best texture conversion tool is paatool v1.1.
3. like all other objects: view -> object browser -> add new -> *select file*
your object will then appear in the object list. use the object placer tool to place them in line. but they must be defined as roads in their config like i wrote above otherwise they flicker ingame.
i'm sure there are more info and some screenshots in the official wrptool manual.
hope that helps