All of the issues you mention seem pretty well chosen. You already seem to know the most important point for getting started with contributing: start small. Take something that is feasable within the time you want to contribute to jbpm. great.
There are some bigger reworks that need to be done with the webapp. I'll post about that on this forum next. But your issues seem to be separated from the rework that needs to be done. So i don't see a problem there.
Make sure a jira issue is assigned to you before you start contributing.
Let us know how it goes.