Help with wiring new dimmer switches
First off -- nice wiring diagram!

To answer your actual question, though:

Putting the dimmer in box A and the remote in box B is indeed your best plan -- it's possible to do it the other way around, although that is slightly outside the scope of this answer.  To do that, you'll need to make the following connections (with the breaker off, of course!):


Dimmer BK pigtailed to the junction of 14 and 18
Dimmer YL/RD connected to 13
Wire 12 connected to the 11/17/15 junction, and dimmer WH pigtailed to that same junction (you may need to upsize the wire nut a bit here)
Dimmer RD pigtailed to 10 and 16 (which are joined together now)
Dimmer GND (green) pigtailed to all other grounds in box A -- if there aren't any bare or green wires in the box, do not connect anything to this terminal
Remote BK connected to 3
Remote YL/RD connected to 1
Remote WH connected to 2
Remote GND (greeen) pigtailed to all other grounds in box B -- if there aren't any bare or green wires in the box, do not connect anything to this terminal
The existing 5/6/8 junction must remain intact here (the 12 to 2 white wire provides neutral to the dimmer remote itself to avoid a NEC 300.3(B)/310.10(H) violation due to wayward neutral current from the remote)
7 and 9 are nutted together to power the lamps on B side from the dimmer feed coming from the A side (the Leviton smart-dimmers need to be on the load side of a multi-way setup)
Finally, make sure 4 is capped off with a wirenut as we have no clue where it leads.  Connecting it to the hot in box B would be a 300.3(B)/310.10(H) violation due to its power coming into the box on the 14 to 3 wire and its neutral returning back from the box on the wire that connects to 11 in box A, which is either 6 or 8.  Connecting it to the dimmed hot in box B would also risk a Code violation if it leads to anything that's not a light fixture, as 404.14 prohibits connecting a dimmer to any load, other than a permanently installed incandescent fixture, that it's not rated for, and 406.15 prohibits dimming standard receptacles.