Answering my own questions as I go: Basque Grand is ~a bad guy from the very start~!
Lab 5 was especially designed to make the Philosopher’s Stone! Check out the size of the room! The carvings built into the wall! It's clearly a Big Alchemy deal, with Father's stamp all over it.
(Transmutation about to begin, chapter 59)
Marcoh was the 'inventor' for this era's iteration of the Stone - the one with all the research. Lots of alchemists and scientists worked on it, and became ingredients for later stones, but Marcoh escaped.
It's not a known process that they're continuing. They're 'pioneering' it (with help from the homunculi).
(Marcoh, Lust, chapter 59)
So in this earliest iteration - in the lab specifically built to make the Stone, the human ingredients are Ishvalans:
(Ishvalans on the transmutation array, 59)
Which Ishvalans? Look at their clothes - oh! Those pants look like military pants!
These Ishvalans are the Amestrian soldiers arrested for 'conspiracy' in the previous chapter.
(Ishvalan soldiers dishonourably discharged and arrested, chapter 58)
There's a level of wear and tear on the building that makes it seem less new, but how long has Grand been around? How high a rank would he have to be to order it built?
Here's some of the wear and tear:
(external wear - concrete breaking off and exposing brick underneath, cracks in the building; internal wear - broken and corroded pipes, bent and damaged-looking metal, chapter 59)
The external looks old. Uncared for. It could be an older building that was repurposed? The internal could be alchemical damage left unmended and given time for some corrosion to set in?
However old the building is, Basque Grand ordered this Philosopher's Stone lab to be built, and he put it next to the prison in order to use them as ingredients. Possibly knowing that they'd be Ishvalans, and he'd be using his fellow soldiers for a weapon to be turned against their people.
All of this before he appeared to be the 'good guy' by shooting the irate, war-hungry, deathmonger Fessler who was undoubtedly on the side of Father's conspiracy.
(Hughes and the soldiers who witnessed this welcome Grand aboard as their new leader, chapter 60)
That means Grand's action there was not to save the Ishvalans and give them a chance to beg Fuhrer Bradley for surrender - he knew they were going to die. And yet when they approached in surrender, he decided to take them back to Fessler. Fessler demanded their extermination, looking like the bad guy, and Grand got to shoot him and seem like the good guy. Even going on to indicate to the Ishvalans that he wanted peace:
(Basque Grand is manipulative or heavily conflicted, chapter 60)
Basque Grand is the Good Cop side of Father's conspiracy, pretending to be just like all the other soldiers - we all want the war to end! - even as he wreaks destruction.
Killing Fessler seemed good to the soliders. Seemed like an act reaching for peace, but all it did was get Grand a promotion, and stop someone else from taking the same opportunity.
