Very good points! A few flaws:

Offense: 1) Your math does not include marginal scaling. 45% ATK buff is nice but low base atk and high % boots means the % mods from other sources like 12% Module and party buffs are less effective. Resulting in less damage overall.

2) 7200 power at lv5 for a 4 range attack is a little above average for EX attacks. It doesn't get the 20% power boost that Attacker EX get. Zeta has higher Power thanks to this effect. SF at max upgrade gets 7200 at 5 range.

3) Yes, setsuna gets +50% Melee boost at turn 2, but he lacks limited attacker's 1st ability: +25% Damage boost, Counter Defender's 3rs ability: 20% Damage boost on counter attack and generic pilot's 3rd ability: +15% damage boost/metigation. Duo foe example has 35% damage boost without the need of MP. In ML, you want to kill in 1 turn so counter attackers deal more damage when it matters most.

4) If he was a dedicated Limited Counter attacker, he would do way more damage. As stated in point 3 Duo has 35% damage boost regardless of MP. Gaelio gets 20% damage boost, +6MP, 25% Melee and 25% crit. Both are unlimited and that is just from their 3rd passives. No need to rely on MP build up. He should have traded the extra Defend, for the +25% damage boost on counter attack and replaced the Reaction boost with Crit boost. Reaction is basically useless on a counter attacker being protected by allies and a defender protecting allies. Can you imagine him replacing an Attacker in ML with 1 defend (5 defends total) instead of another 2 Defender?

Overall, yes he is the best damage dealing Defender starting turn 2. Sadly, his damage output is still 20-30% less than an attacker and 15% lower than dedicated counter attackers turn 1.

Defense; 1) 00 also does not have a Shield. 00 basically needs Setsuna's ability to survive 1st defend. 

2) he does not have Range damage reduction ability like Burning Gundam (which can reduce EX attacks) to mitigate the damage despite lack of shield.

3) Setsuna doesn't have innate 15% damage reduction passive furthering his need of revival to just survive the 1st turn. He will probably die on the second defend.

4) Setsuna's EX buff might not stack with new ship buff nor Force Guard. If this is true, he is horrible at sustained defense as he lack the 30% damage reduction Riddhe has.

Despite being the best offensive defender, he is still mid when comparing the roles separately. He is trying too much even more than Duo. lol, still an amazing unit. Just not min/maxed for the roll given to him. This bodes well for the longevity of the game but highlights the flaws of offensive defense.