I am taking as an assumption that she’s a Null, and using what affects her to define the bounds of what a Null can be.
The pure Naught bloodline was too good for me, supposedly. But all it did was keep ‘er alive a little longer after she lost Paimon.
Naught. Perhaps a variant on “Null?” He had heard rumors, of course, about those who were born with the traits of a Null, yet still able to cast magic. How they could resist the madness that came with casting through their own flesh and blood.
Or, perhaps the Blood Emperor’s experiments had not been so fruitless, after all. This changed everything.
Raaz Kalvidasan, Siobhan Naught’s adopted grandfather, may have had some connection to the Third Empire’s cohort
I guess you can suppose Lacer is wrong here? But the thing about Miakoda, and the later reference to the Blood Emperor’s experiments being successful + Raaz’s origins, suggests pretty strongly that he’s not. She is a Naught, i.e. a Null that can cast magic.
The question then is what being a Null gets you. You seem to be assuming that it makes you immune / resistant to healing, stunning, etc. I am not assuming that.
I think they’re called Nulls primarily because they can’t cast magic, not because they’re immune to it:
Even Nulls, who by some anomaly of genetics could not cast magic at all,
I think we agree Oliver is a Null.
Magic was even in the carpet beneath his feet, an illusion spell mimicking new grass. It was hard not to consider his own differences in a place like this.
What affects Oliver?
The healing potion spread its magic throughout Oliver’s chest, but, despite its potency and commensurate price, he could feel it petering out against his natural resistance before it made it much further. One of the many curses of his bloodline.
In a blur of confusion, Oliver belatedly attempted to leap up and over the foggy force and wave of wooden shrapnel. The blast clipped his shins painfully, sending him twisting through the air.
Lord Morrow stared at him in surprise, looking back at the intact ward. “B-but that’s impossible!” Obviously, he had assumed that the ward, keyed physically to him, would keep Oliver out. If it were someone else, perhaps it would have.
Those quotes clarify a bit about what a Null can and can’t do: they are resistant to some potions, not noticeably resistant to battle spells, and are able to walk through some wards (clearly not all, or Oliver would have done that in the first place). At Knave Knoll he is also affected by a calming spell and the air witch’s spells. Nulls are not immune to all magic. They are not even, seemingly, resistant to all magic.
There’s a more explicit explanation in a more recent chapter:
Nulls were at a disadvantage in many ways, being entirely unable to do magic, but they had a few specific advantages. Because of the way that magic struggled to affect them, they were useful in combating Aberrants and countering magic that lacked physical components.
Of course, the Architects might outfit them with artifacts and use them as a fighting force, but the problem with the most common battle spells was that they caused harm by affecting the environment. Oliver was resistant to magical effects, both good and bad. But he was just as likely to get roasted by a fireball or have his organs ruptured by a concussive blast as anyone else.
So Nulls are resistant only to direct effects of magic. And they are only resistant, not immune.
Siobhan clearly shows some of the other Null traits:
She slowed as she was forced to push through the magical resistance around the doorway. Finally, she slipped past as if the barrier were a thick, invisible soap bubble that snapped back into place behind her. Kiernan let out a strangled sound from behind her.
She is mysteriously the only one not rendered unconscious by the Pendragon corps’ stunning mine.
She is the only one to resist Lacer’s conch shell evacuation spell without help.
She is able to mostly ignore her vow to the Red Guard regarding the Newton incident, and tell Damien and Oliver about it.
In a more recent Patreon chapter, she was the only one mysteriously unaffected by Lacer’s powerful spell.
I think there are usually too many confounding factors to say whether she’s resistant to potions or battle spells. There’s some evidence in that direction:
The potion had been too weak, or she’d sustained too many injuries, to fix everything. She could feel it tugging futilely at her abdomen
Plus I am assuming by default that there is a gradient of Null bloodline strength, and that some might have stronger or weaker powers. So Siobhan e.g. not being resistant to potions in exactly the same magnitude as Oliver isn’t surprising to me.
But I think being a Null is subtle, unless you fully can’t cast magic. The divination diverting ward is probably making it even harder to detect, since it offers an expected reason why people meet strange resistance trying to target her with a lot of spells. Plus, being a magic-casting Null is supposed to be impossible. I think people would go to a variety of more likely conclusions first.
Anyway, all of the above together makes it pretty clear to me.