Never tried shoe polish, but the color may not match and look worse than before. I have zero input on this one.
As far as softening the leather, yes there are some products out there that really work well. Leatherique would probably be your best bet. But this product needs to sit for a while to work properly. When I put it on my truck seats, I usually let it sit for about 3-4 days before removing it. For some people this isn't possible because you can't really be without their car that long. The longer you let it sit, and the hotter it is, the better. Apply, park it out side with the windows up for a few days, and you'll be amazed at the results.