Ahhh! There is my problem with the back side repair. After all the time spent restoring a canoe, those gawdawful cracks are left staring you in the face through layers of new gleaming varnish. I have used the method before way up under the gunwale like your bad spot and I would not hesitate to use it if I were you. I think modern epoxy makes the butt joint less of a problem. You can also play with the shape of the mortise/spline to make it less of an issue. Another fix is to scarf a new rib tip on. This removes the offending crack (you are left with a nearly invisible glue line) and judging from the photos, you would not lose much rib? There are lots of tutorials on the forum about scarfing rib tips.