Edited by Shouldaville - 11/3/15 at 3:09pm
The oil tan leathers are by far the most durable, but are not very prevalent right now. They are the more traditional workboot leather. But also of course harder to break in and heavier.
If you're interested in what the leather looks like, it looks like this. These are from Notre and they're nearly identical. I think the only difference is these have a dainite sole and unstructured toe.
I can't see that improving with age. I'd try a thin insole and see if that makes a difference. Also, make sure they are laced up tightly.
It depends on the sole, not sure what soles the OP has. I've never tried a tongue pad, don't think I'd like that much. I also don't like heel pads - they never stay put. It really just depends on the individual foot and the shoe/boot. I have had some luck in the past with raising the foot slightly (which is why I suggested a thin pad), just enough to make the shoe a bit more snug and hit that sweet spot in the heel area. It's certainly worth a try. Having said all that, heel slippage is one of the most challenging areas to correct. Not impossible, but length is a lot easier to deal with.