i've also seen that before applying mortar, you should prime the subfloor? anyone heard of that?

You do need the thinset under the CBU per manufacturer warranaty and because it is the correct thing to do for the life of your tile. You dont need any primer. You can use a cheap thinset under the boards because the purpose of the thinset is to fill in any air pockets and voids between the CBU and the plywood subfloor. This eliminates a major source of flex in the floor which can transmit up to the tile and crack them. Backerboard screws or 1& 1/4" galvanized roofing nails do the work of holding the CBU down, not the thinset under them. Also, you want to tape the seams with mesh tape and apply a coat of modified thinset in the same manner you would apply spackle to wallboard tape.