There's a big difference in the workability and strength of brick mortar and deckmud, and you'll find that brick mortar over a curb can be installed much easier and does not need to be anywhere near as thick as deckmud. If you use deckmud, you'll end up with those huge curbs John hates. If you're going to use that technique, and you've got your studs laying horizontally, you'd have a curb (without tile) 6.5" wide if you use the thickness John states you need using deckmud. That's thicker than the Kerdi foam curb John has stated he hates (because of its size). Brick mortar would be unsuitable for the setting bed...it is a bit sticky and does not have the porosity required to allow moisture to migrate to the weep holes...that's not a big concern on your curb.
If you want professional, unbiased, help with your shower, I really suggest you try out
www.johnbridge.com. They have hundreds of pros there they can tap into to answer questions like this, and they have a significant library (they call it a Liberry) of industry references, how-to's and guidance that will answer most of your questions.
Or, you can put up with the distractions John Whipple throws in to try to discredit anyone else's suggestions.