Will A Little Soap With Water Kill Plants?

Well I have found that even a bit of soap water will build up over time. The soap doesn't get absorbed naturally by the soil and will soon become a thick layer of soap residue. This will block  water from being properly absorbed by the soil and make it harder for you to water them.

Yes it will
I did a little experiment and nothing has grown. Instead the seed has molded

Yes it will the plant will still grow but very slowly

Yes it can kill plants.

Most likely. The chemicals can prove themselves to be fatal, and can also dry out the soil and plant.