You can use the integration api as defined in the gwt modules (which would be a very good idea if you actually want to offer these APIs using a REST service), or you could just use the underlying api to get the information you need (just like the gwt implementations are doing). But the gwt implementation is a fully functional example, so you can definitely take a look at that.
There are numerous APIs that you can use to do what you are describing. I would suggest looking through the documentation; examples and API to see how they are done. Some pointers:
Everything related to loading process definitions:
KnowledgeBuilder kbuilder = KnowledgeBuilderFactory.newKnowledgeBuilder();
kbuilder.add(ResourceFactory.newClassPathResource("BPMN2-MinimalProcess.bpmn2"), ResourceType.BPMN2);
KnowledgeBase kbase = kbuilder.newKnowledgeBase();
Creating a session:
StatefulKnowledgeSession ksession = kbase.newStatefulKnowledgeSession();
Starting a process instance:
ksession.startProcess(processId)
Everything about monitoring your process instances, use a history logger:
https://hudson.jboss.org/hudson/job/jBPM5/lastSuccessfulBuild/artifact/target/jbpm-5.0-SNAPSHOT-docs-build/jbpm-docs/html/ch07.html#d0e1687
Everything related to task management, use a task client:
https://hudson.jboss.org/hudson/job/jBPM5/lastSuccessfulBuild/artifact/target/jbpm-5.0-SNAPSHOT-docs-build/jbpm-docs/html/ch.Human_Tasks.html#d0e1446
If you have specific questions, just let us know ;)
Kris