If it were me, I'd adjust the grade of the landscape bed (within 3' of its edge) so that it was flush with the surrounding grade. It should not have been raised up above the surrounding grade. The edging you've chosen will not look like a professional solution. It is definitely in the homeowner-installed category. Mortar would not be necessary. Presumably, you'd be sinking it 50% of its height. It would be fine to set it on a 3-4" layer of tamped paver base (or any crushed material that packs tight and doesn't have pieces over 1/2" dia., as then it would be more difficult to work with.) Mortar is not necessary and would probably backfire when any changes were desired. It would probably not hold better than sinking the edging sufficiently.

Thanks!

Well, we actually chose a different edging today. We were going to buy the original ones, and found a different style that looks more like a natural stone, but can be layed on edge. We both liked it better, and it's a better match to the house brick.

When we converted from grass to landscaping, the grade WAS even with the driveway. We've just lived in this house a few years, and we were told by the neighbors that the berm was added several years before that.

It appears that the mound of soil is creeping it's way towards the driveway. We certainly could regrade that leading edge, but I'm not sure how long that solution will last.

Is gravel neccessary, or can we just set them in the soil?

If you set them directly in soil, expect their level to shift over time. It will not look good.

If you think that the edging is a solution to the berm creeping toward the drive, you'll find that it's not. It's just a delay. Since it's a bed, you could periodically adjust the soil level at the 3' swath near the edge and not need any "edging."