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My mistake, your Flurry of Blows is only 1 Ki point. For some reason I had 2 in my head.
As for the Stunning Strikes, I would change my ruling in your favor if you had complete information. I'm not sure the DM is obligated to explain his rulings (and I take no offense with your argument), but here is how it played out. Haun had cast Mirror Image with his first Legendary.
Mirror Image
Three illusory duplicates of yourself appear in your space. Until the spell ends, the duplicates move with you and mimic your Actions, shifting position so it's impossible to track which image is real. You can use your action to dismiss the illusory duplicates.
Each time a creature Targets you with an Attack during the spell's Duration, roll a d20 to determine whether the Attack instead Targets one of your duplicates.
If you have three duplicates, you must roll a 6 or higher to change the attack's target to a duplicate. With two duplicates, you must roll an 8 or higher. With one duplicate, you must roll an 11 or higher.
A duplicate's AC equals 10 + your Dexterity modifier. If an Attack hits a duplicate, the duplicate is destroyed. A duplicate can be destroyed only by an Attack that hits it. It ignores all other damage and Effects. The spell ends when all three duplicates are destroyed.
A creature is unaffected by this spell if it can't see, if it relies on Senses other than sight, such as Blindsight, or if it can perceive illusions as false, as with Truesight.
On your first Stunning Strike I rolled higher than 8, which changes the target to a duplicate. Your attack roll was high enough to beat the target AC, so you hit with the attack and destroyed the duplicate.
You declared "Spend 1 Ki point per attack that hits, up to 4 Ki points if needed, to force a CON save vs stun, as per "Stunning Strike". He will stop spending KI points for stun check if/when one succeeds." The attack hit.
3 Ki points used. In almost every other case other than by this spell's mechanics, you would be justified to challenge the DM there. Haun beat the ConSV with his awesome modifier, so I flavored it as him staggering. He was not stunned although he did take the damage from the second attack. I am happy to post the creature statblock and spell list after the encounter.
Abi's turn.
edit: After looking at Stunning Strike again, I think the order of operations in regard to choice is in a very gray area. I am changing my mind and ruling. When the hit happened it dismissed the target at the same time Renda would have had to make the choice to use the Ki point, which he would not have (but the hit did land).
2 Ki points used.
Starting at 5th level, you can interfere with the flow of ki in an opponent’s body. When you hit another creature with a melee weapon attack, you can spend 1 ki point to attempt a stunning strike. The target must succeed on a Constitution saving throw or be stunned until the end of your next turn.
As for the Stunning Strikes, I would change my ruling in your favor if you had complete information. I'm not sure the DM is obligated to explain his rulings (and I take no offense with your argument), but here is how it played out. Haun had cast Mirror Image with his first Legendary.
Mirror Image
Three illusory duplicates of yourself appear in your space. Until the spell ends, the duplicates move with you and mimic your Actions, shifting position so it's impossible to track which image is real. You can use your action to dismiss the illusory duplicates.
Each time a creature Targets you with an Attack during the spell's Duration, roll a d20 to determine whether the Attack instead Targets one of your duplicates.
If you have three duplicates, you must roll a 6 or higher to change the attack's target to a duplicate. With two duplicates, you must roll an 8 or higher. With one duplicate, you must roll an 11 or higher.
A duplicate's AC equals 10 + your Dexterity modifier. If an Attack hits a duplicate, the duplicate is destroyed. A duplicate can be destroyed only by an Attack that hits it. It ignores all other damage and Effects. The spell ends when all three duplicates are destroyed.
A creature is unaffected by this spell if it can't see, if it relies on Senses other than sight, such as Blindsight, or if it can perceive illusions as false, as with Truesight.
On your first Stunning Strike I rolled higher than 8, which changes the target to a duplicate. Your attack roll was high enough to beat the target AC, so you hit with the attack and destroyed the duplicate.
You declared "Spend 1 Ki point per attack that hits, up to 4 Ki points if needed, to force a CON save vs stun, as per "Stunning Strike". He will stop spending KI points for stun check if/when one succeeds." The attack hit.
3 Ki points used. In almost every other case other than by this spell's mechanics, you would be justified to challenge the DM there. Haun beat the ConSV with his awesome modifier, so I flavored it as him staggering. He was not stunned although he did take the damage from the second attack. I am happy to post the creature statblock and spell list after the encounter.
Abi's turn.
edit: After looking at Stunning Strike again, I think the order of operations in regard to choice is in a very gray area. I am changing my mind and ruling. When the hit happened it dismissed the target at the same time Renda would have had to make the choice to use the Ki point, which he would not have (but the hit did land).
2 Ki points used.
Starting at 5th level, you can interfere with the flow of ki in an opponent’s body. When you hit another creature with a melee weapon attack, you can spend 1 ki point to attempt a stunning strike. The target must succeed on a Constitution saving throw or be stunned until the end of your next turn.
ah, very good! 
