This is incorrect. You still brush your feet in modern today, as we never take steps that are purely forward or backward - there is always rotation of at least the hips involved, so feet that may be on seperate tracks at the extreme of stride still brush together in the middle.

I think it is alright to practice by yourself. I do it too. I dont have a partner anymore but i still dance around the floor. I don't want my skills to go down because I don't have a partner. In fact my teacher says it is good to practice by yourself too. So I think it is ok.

Yes, though bad habits can develop without the presence of another body to indicate the problem.

For example, swinging the foot in front of the body, rather than moving the body with the foot underneath. Alone, dancing between your feet feels wonderfully supported and stable, but you couldn't do it with a partner there - unless you artificially seperated your leg tracks enough to interlace legs with them. With proper body action, your leg's won't interfere with the partners even if they brush together, because your legs won't extend in front of you into their space.

If you want to continuously knee cap each other stand on two tracks and do those pitiful little srides. Now stand on four tracks take one step and get your front foot within three inches of your partner back foot. Her front foot should be within three inches of your rear foot and a little to the side and on her own track. But then dont believe me. Have a look at the best and see for yourself, and if you can do this and brush your feet your a better man than i am Gunga Din

If you dance the body rather than the feet, you won't bump knees, and you will eventually find your feet brushing as there will be no spurious forces sending them away from your track of movement.