Any of those would work and are approved methods. You can use plastic, if you prefer, rather than the roofing felt. Assuming the tub has a tiling flange, as long as the vapor barrier laps over it, it doesn't actually need to be adhered to the tub lip. If it doesn't have a tiling flange, you might consider an add-on kit. Or, if you decide on Kerdi, you could use one of their Dilex expansion joints at the tub edge (and in the corners, if you want), and not have any need for caulk. Industry standards call for caulk on the changes of plane or materials, but an engineered expansion joint works, too. Keep in mind, though, that the expansion joints really want nice, square, plumb corners. If you don't have those, probably better to go conventional with caulk.
CBU tends to suck up moisture from the thinset, so it's often a good idea to wipe it down with a wet sponge to both remove any dust and dampen it a bit - not wet, just slightly damp.