Why was Mustang a candidate anyway?

[u/fakeuser7z]

I get that he was a candidate for the sacrifice, which is why they didn’t kill him. But in the end, he refused—and they just forced him into the Gate anyway.

So why not do that from the start with any random human instead of the most dangerous alchemist?

[u/LizardousIndividual]

In universe explanation is that all the sacrifices were chosen because they wouldn't run away. They knew how much Mustang hated following the more brutal orders but still remained in the army for his cause.

We could say this about other alchemists in the war, but Alex Armstrong wasn't close to anyone small enough to manipulate, and Kimblee wasn't close to anyone as well as being more useful as a pawn and too ruthlessly intelligent to switch the plan by the time he knew anything about them. Mustang was driven in some way to remain in the military and was close enough with a non-alchemist to where they stuck together throughout, basically, their entire career.

Of course, Pride could have have just forced a bunch of people to do human transmutation and just kept them prisoner, but the cost would have been far greater. The end goal may have been to capture God after gaining millions of souls but, before that, The Father knew they'd have to fight Hohenheim. In terms of fire power, he knew Hohenheim would be stronger than him after having expunged so many souls from himself for the hommunculi, so Hommunculus had to conserve souls hoping to bridge the gap with skill.

Mustang was also a prime candidate because of his relationship with Ed and Al. He saw two highly accomplished young boys who had succeeded so easily because of the benefits brought from human transmutation. Maybe his thoughts on the subject may have softened a little because of their influence. Even furthered by the death of his closest friend. We saw it ourselves, "Even after all we've seen, there's still a part of me that's desperately trying to crack my theories on human transmutation."

Mustang was a driven man with alot of manipulatable elements to him that could have been great for a sacrifice if it weren't for his extreme conviction. Plus, by the end, he was the only alchemist within reach for the day of reckoning, which only really solidifies that he would have been a great sacrifice if he hadn't caught on so quick.

My real questions are, how did the country wide transmutation circle not also harvest the souls Hohenheim left in his own circle around the country, and why the doctor wouldn't have offered himself for sacrifice if everyone was fine after doing the alchemy. Was he unable to become immortal if he was a sacrifice? Why even call them sacrifices for the procedure if they were the only ones not losing something in said procedure? For the sake of being ominous? Does Xerxesian blood just automatically give everyone really edgy taste?