Hi Shawn,
Your cactus flowered specimens look great. You could apply some pollen to them from other specimens to hybridize them.
" I've never seen anything quite like it, in all my years. Nearly all the seeds that aren't even quite ripe yet, are almost entirely gone, by having been eaten by those gold finches . "
I know how frustrating that can be. Saving your seeds at the green stage is one possible solution to the problem. Applying protective sacks or "socks" over the blooms can protect the green seeds.
Or applying protective screens over the entire plant is another way.
That nylon screening is supported by one of my "zinnia cages", with attachments between the screen and cage at a few key spots with safety pins. You could use a tomato cage as a zinnia cage, or maybe just some sticks inserted into the ground.
So an offensive strategy against seed-eating birds with BB guns or pellet guns is out, but a humane defensive strategy based on netting fabric or nylon screening, supplemented with early seed saving at the green-seed stage, can give you a decent yield of cross-pollinated seed.
Incidentally, I make those protective net "socks" out of netting that I purchased from a fabric store, and I just used black yarn and a big knitting needle to thread the yarn to join the edges of the netting. A zig-zag stitch with a sewing machine or serger might be faster than the knitting needle, but I take long stitches with the knitting needle, so the netting joinery goes pretty fast.
ZM
