I have never used sea salt and not sure what it is to give a good answer, do you have kosher salt, what you want to avoid is the anti-caking, the iodine in low doses is usually not harmful to salt tolerant fish
What is wrong with your Betta
Yes, and they are a pretty easy egg scatter- but will eat the eggs as fast as they are dropped.
I use a 10g-76-78F-1-female and 2 males with a layer of marbles and java moss for the eggs to fall through to keep them from eating them. I use a mature tank and sponge filter, I make a large water change using cooler water the night before and the first thing in the AM when I turn on the lights they start to spawn, I remove all 3 adults after about 2-3 hours or if they stop spawning.