First, for raid defense, as long as you are filling your hero chest in Diamond then how many cups your D holds is just for ego, so I don’t recommend getting too worked up about it. War defense is somewhat more important, but if you aren’t in an alliance that has its stuff together then it doesn’t much matter how good your war D is either! Having said all that, here are some thoughts on your actual questions…
You don’t need a healer on your raid D. If one fits, that’s fine, but I wouldn’t sacrifice power just for healing. As for your specific lineup, I don’t like healers sitting in the wings unless they are going to bring heroes back from the dead, so I’d drop Rigard for Kage. Since Kage’s job is to hit the enemy really hard early and often you want him in a flank spot, not the wing.
If you want to run Justice as your tank then that’s fine, but she isn’t ideal outside of that role, and you definitely don’t want her in the wing. Since she would plunder emblems from Frida, who is more generally useful, I’d max Joon ASAP and install him in the wing, next to Kage.
The rest of your lineup is fine for the time being, and I would think you would have no problem filling all your chests in Diamond, but ultimately you can improve it a bit down the road. Azlar will hold his spot ok for now, but he’s the next candidate for replacement, in my eyes.
You have two great red heroes in Grazul and Black Knight. Grazul is a monster of a support hero on offense, but I find her more of an annoyance on defense than anything else. Black Knight shines on defense and is the best pure red tank out there, in my opinion, and from what I can tell he is pretty handy on offense as well with his mega team attack buff. Each hero is worth ascending, but each has an emblem problem. I generally like to focus my emblems on my D team, leaning toward my war team if I have to choose, but I think Rigard is a hugely useful hero and I really wouldn’t want to strip him for Grazul if I didn’t have to. Black Knight competes with Kage for emblems, and you don’t want to have multiple heroes of the same class on you defense if you can avoid it (assuming you have finite emblems, that is). So what are you to do? I think that for defense you are probably better off with a Black Knight without emblems at tank or in the Azlar flank spot next to Frida and focusing your barbarian emblems on Kage. another long term option would be to max Sartana if you don’t have another wizard of higher value and pump her up for the Kage spot so BK can maximize his emblems, but Kage is really strong and it would take a whole lot of wizard emblems to make Sartana definitively better. All that being said, if you are tight on rings (as most of us are!) I don’t know if maxing Black Knight over Grazul is the right or wrong choice, but you only asked about your defense team!
Hope some of my ramblings help!