You should get soil for seedlings or a really light soil with no nutes in it, and then transplant it into the soil with nutes in it when you're ready.
To flush a plant is to take 3x the amount of water that that you would normally water you plant with just keep pouring the water in and let it run out the bottem holes in the pot until you used up the 3x amount of (just) water.
Your plant won't die, it's not that sever of burn but it don't help it.
yes sir, you have to have drainage hole in the container bottom. i don't start feeding until about 2 weeks. if you started that in pre-ferted soil, then that'll cause problems. i'd x-plant to a better container and use some other medium.