Putting oil or conditioner on the bottoms will make her feet softer. I would use Hoof Heal on just the outside of the hoof 2-3 times a week during dry months. Do not use it more often than that.
Ask your farrier/trimmer to show you how to touch up your mare's hooves between trims. You want to round the edges off when you see them growing too long or flaring out. Your farrier/trimmer should be able to show you what to look for and how to correct it enough to prevent chipping between trims.
Make sure your mare is on a 4-5 week trimming schedule during the spring, summer, and fall to prevent chipping and cracking.
As for toughening the bottoms, I would use Durasole and work your mare on rough/hard ground as often as possible, starting slow/easy and working more as she toughens up. If she is stalled, get her on as much turnout time as possible, preferably in a paddock/pasture with some rocky/hard areas that she'll have to go through.
If she just won't toughen up, ask your farrier/trimmer about Hoof Armor as a possibly protectant for the bottoms of her feet.