With any Kerdi joint, you want a 2" overlap, in this case, to the niche material. If you can't get that, then I'd want to seal those edges with the KerdiFix. As long as the dryset mortar is a quality one, and I don't consider some of the entry level, price leader stuff quality (they may meet the minimum spec, but they don't spread or work as well as the better stuff), the Kerdi should be fine if you follow Schluter's instructions. Because the Hardie material is VERY thirsty, to keep from sucking too much water out of the thinset before it gets a chance to bond (remember, thinset cures, incorporating the water into its chemical structure, it doesn't dry), you want to wash the dust off the Hardie and go over it with a damp sponge just before spreading the thinset. This is more important in the winter when the air tends to be drier, too...the combination of dry air and dry cbu, can compromise the bond. After installing, especially until you get the hang of it, pull it back and check the coverage - it should have a full coat of thinset embedded into the fleece. If the consistency is correct, it will then lay back down and you can work out any excess mortar to re-embed it well. You should not be able to see trowel notches after embedding the stuff - it will have a fairly consistent color all across the material. Dont' try to work with too big of a piece at once, the thinset will skin over before you can embed it properly. IF you're using the Kerdiband, you can install that first, or over the sheets - it doesn't matter. I think it's a little easier to do the seams with the band, then install the sheets. Deal with the corners either before or after, but get the seams in first. From a waterproofing standpoint, as long as the seam overlap is good, it really doesn't matter, though, and you may find you like to do it after. I think it's easier to get nice clean corners with the band, then you can run the sheet closer to the corner.