Yes, the lvl 91 14.96" guns quadruples on the front slots. On the rear R-slots, I'd only put Hedgehogs. At the moment, they are not that useful, as there are no subs ingame. But after the event...

I see most Richelieus without AA, but if you're good at it, put the standard French AA guns on the T-slots (the lvl 20 3.5" doubles, they are excellent).

To answer you directly (before I begin my rant) the rear R slot you must control and isolate using the Z and C buttons to select front/rear R turrets. The back turrets can have 4x3.5"/50Cal AA guns to snipe down scouts, or 4x5(7) depth charge launchers. I find sniping down scouts more beneficial to your team, but you'll always be cross with yourself when a sub is chasing you lol.

And my final advice would be to ditch the Richelieu all together because the ship is outranged by BB3's and just keep with the Dunkerque until EXP becomes unbearable. Because Richelieus range is just appalling.