1: jBPM supports 'subprocesses'
2: please define what *you* mean by application layer and businesscore? and why you think they are not separated. Btw. the jBPM console is meant for RAD and Demo purposes, not full blown applications.
3: Yes, you can use any presentation layer. jBPM has a fairly simple api to use.
4: WS frontend? Not yet... backend: just implement it in java like you would in a normal pojo