wouldn't jacking up the lower A-arm reduce the preload better than the spring compressor? or am I missing something?

My spring compressors that I used when installing are the same one's in your picture of what not to use. They worked fine but I was certainly on edge during the process.

On a side not, those were too big for my wife's Libby (KJ), so I had to buy a cheaper autozone set that kept on snapping around and bumping into each other.... never came off, but quite alarming and you have to start over each time as the spring is bent in a U.