Are the soles memory foam?

Yes they are. My husband wears them to work everyday and loves them. Only boots he'll buy.

Anyone warranties for horseback riding?

I am not sure what you mean by "warranties", but I see no reason these would not be fine for horseback riding. The heal is not as tall as a cowboy riding boot, so it may not stick in the stirrup as well a boot made specifically for that purpose.

Are these easy to put on and take off. I have a bad back, so bending can be fairly painful.

These are probably not for you. It takes a good tug to pull them on because the ankle is fairly tight. And you certainly can't slip them on without bending over.