It's like you saying, "Dogs and alligators both have four legs, so they're basically the same," and then cleaving to that point on matter what. Teams aren't the same just because they have same records. Why aren't they more like Portland, to whom they were just as close as they were to Sacramento? I doubt you'd equate those two teams and their make ups, but because the Kings are the more dire position, you'll force yourself to just see them.
SGA is hindsight. There were reports they wanted to trade into the top 10 for Walker. That would definitely suggest they have Lonnie on their board higher. You're correct that it's not guaranteed that those guys were not also in their top 10, but there's also no guarantee that Keldon is going to be worse than whomever the Spurs would've taken at 18 had they already have Walker at 10 or 12/13. Assuming the Spurs would not have drafted two two-guards in the same round, they'd be looking at guys like Huchison or Wagner. It doesn't seem to bad that they got Johnson instead.
And yes, DeRozan has limited value, but he had about as much value as Kawhi did at the time of the trade. That's why Toronto got Green, took bad no paid money, protected the out of their first and had the Spurs pay them the max allowed to cover the Kawh's kicker. Teams immediately called the Spurs trying to trade for DeRozan, and the Spurs said no. Maybe none of those deals were good, but they likely could've picked up at least an additional pick had they wanted to, in the same why it seems like they might be able to still have another first this year if they want.
I know you're the king of thinking the worst-case scenario is always going to happen, but this isn't a butterfly-effect thing. Had the Spurs won that game, they would've gone into the final regular-season game against a resting Utah with a chance to make the play-in game. They pretty comfortably beat Houston and NOP. There's no reason to assume different outcomes there. If anything, their loss to Philly probably hurt them against during the next game against Denver.
"Adding" is referring to Johnson, not moving up in this draft. Johnson directly did come from the Leonard trade. If the Spurs move up, get a blue-chipper and add that player to Johnson, and come out of this with cap space and young players, it seems like what you wanted them to do but also with them having had chances to "go for it" for two years. Trading for DeRozan gave them that flexibility, and apparently they still have it by your own admission. Yes, maybe they could've found a way to get a higher pick than 19 last year (I think they could've just by trading Poeltl, but that never happened) and targeted Hachimura/Doumbouya. You can argue though that taking Clarke at 10-14 would've been the play as much as taking him at 19, and if that were the only pick SA had last year, they would not have been able to swing around and draft Johnson.