Congrats on finishing your paddle! I like your attitude about experimenting - for me it's part of the fun of the hobby.

You may notice the paddle has a slight arc. No idea how this happened; I measured everything before I started and kept re-marking with pencil. I suspect the board wasn't as straight as it appeared. Or maybe I flubbed the planing.

It's pretty common for boards to change shape once you start removing material. It can be that the board isn't evenly dry through its thickness, or that there were some stresses 'locked into' the board during growth which rear their ugly head once the machining starts.
Roughing out and waiting a few days to continue is a common strategy.

If it's a drying or moisture equilibrium issue, it's a good idea IMO to support the board (throughout the project) so that all surfaces are open to the air.
More than once, I've planed a board and left it overnight on the bench to find it 'warped' next day...nothing that another spell of 'airing' couldn't correct. Also, these moisture issues can arise with very dry wood and a more humid shop environment.
So, it wasn't necessarily any mistake on your part, or even a 'bad board'....

Use latex gloves, because epoxy WILL get all over your hands, and it’s hell to clean off.

You definitely need to keep epoxy off your skin to avoid sensitizing yourself and developing a serious allergy. Gloves and long sleeves (and pants, and socks) would be the minimum protection. Also be careful 'cleaning off' epoxy - using solvents will remove epoxy but can help to increase your cells' exposure. No doubt others will have suggestions for removal strategies - one used to be vinegar.

Use a very sharp blade on your hand plane.

I'd direct my efforts toward sharpening (skill and technology) before spending a lot on a new plane. If you have a Stanley/Record plane, you can buy a better blade as an upgrade, but you still need that sharpening skill to make it perform.
Veritas (LeeValley) is another good brand of plane.

So I added to the resin and hardener a dollop of white, water-based acrylic art paint that I had lying around.

Dry pigments from the art store work well when mixed into epoxy, but they are as expensive as the SystemThree pigments (ground in epoxy resin). (Some of those art pigments are nasty if they end up in sanding dust, so be careful..)
I'd do experiments before mixing other liquids into epoxy.