What I've noticed is that a cedar 2x4 that's supported just in the middle will sag at the ends, just from it's own weight. If you flip it over, it will straighten out, then sag in the other direction. Cedar is often soaking wet when you buy it. I always let board air dry for a couple of weeks or more (supported at several points along its length) before laying out a paddle. If you draw centerlines on all four sides and do your layout based on them, you'll get a straight paddle out of a warped board.

Acids like vinegar neutralize epoxy. Since citrus cleaners are also acidic, I'm not surprised that it worked well, too. Soap and water cleanup afterward is standard practice, so you did the right thing all around. BTW, this is why epoxy isn't recommended for wood like oak (red oak especially), since they contain enough tanic acid to cause problems.

There's not only nothing wrong with putting it on thick, it's beneficial. The epoxy is supposed to be a sacrificial layer that will wear in use and be renewed periodically. Having it slightly proud of the surrounding wood helps to protect it.

Abalone inlays might be nice for a "wall hanger", but they're not practical for a working paddle. They're heavy and they'll get destroyed quickly in use.

You seem to be overly focused on aesthetics at a time when your main emphasis should be on the design and construction of good, functional paddles. That's not to say that you can't have both - you certainly can - but the prettiest paddle in the world is useless if it doesn't perform well on the water or it's not comfortable to use.

I recommend that you refocus on building your woodworking skills first (including sharpening, layout and working to your layout), as you need that as a foundation. Learn what you can from this first paddle, then move on to the next one. You're moving in the right general direction, you just need to adjust your course a bit.