Argh! I forgot about the dihedral brace running through there. Well, if you epoxied the dowel plate in you should be able to remove it with careful use of a covering heat gun. What you need to do is use the heat gun to heat up the epoxy, then scrape as much away as you can with a hobby knife. Keep heating and scraping and you should eventually be able to loosen and remove the errant piece. Make sure that the inside faces of the spars are clean of hardened epoxy, as the dihedral brace is a snug fit between them.

If you used CA to attach the dowel plate then liberal application of debonder should fix the problem.

Good luck,
Neil.