I believe CA-JL do have an interline agreement. If your flights are all on one ticket, CA should be able and willing to interline to JL and get your stuff all the way to NRT. Worth asking them at PEK.

If not on the same ticket/PNR as the JL flight, then I doubt they'll be willing, and you'd need to manually claim your bags in TPE and do onward recheck.

Usually with CA, you need to call them by phone, and get seat assignment. Or get to PEK check-in desk a bit early and ask for your preference. Most Chinese don't bother with advance seat assignments, they just accept what the check-in agent gives them.

Don't know about the JL BP issue.