Oh no! I'm so sorry
As with any living thing, sometimes you do all the right things and it isn't enough. And it's hard. You know, I just don't recall my fish from when I was growing up having anywhere near the personality that bettas do, these guys (and gals) really wiggle their way into your affections, and stay there.
But you are 100% right, it's better to know and take the right steps. Or even just take steps based on a good guess, because doing nothing might have worked out OK for the scale (might have taken longer to heal) but might have had a bad result if it had been something else. Which, given the swamp soup the algae tab had turned his water into at the start of all this, was a very real possibility.
Thank you so much for your guidance. Indigo looks like he's ready for transition back to normal conditions, and he thanks you too!