Concrete pavers can work well for a curb.
the idea of the second layer of ply is partly to decouple, partly to strengthen, and as a result, it should be installed offset from the first layer with no joints aligning and not into the joists.
The reason for thinset underneath the cbu is to fill in any imperfections to provide 100% support, not to hold it down, the fasteners do that. Most, not all, specify unmodified, but a modified will work as an alternative. Some specify a modified...basically, download and read the manufacturer's installation instructions.
A sample of two: HardieBacker and Wonderboard do not say anything about filling seams with caulk. Wonderboard does specify filling the gaps with mortar prior to installing the tape. At least on the backer, the idea is to make it a monolithic layer, the mortar and the tape help tie the individual sheets together. While filling any gaps in the ply with caulk can't hurt, none of those I've looked at specify it. If they felt it was that important, not wanting their product to fail, I think they'd specify it. Can't hurt. From what they told me in class, was that over time, the subfloor moving will cause the backer to crush around the fasteners into a powder to allow for some movement between the tile/backer and the ply underneath.