"Alright, forget about legendary family history for now," you say, looking at Ambrose. "Before she agreed to talk to you, Tatsuki seemed pretty determined to have that seal off. For the sake of argument, let's say she finds somebody willing to do that. What would it do to her?"
"In the absolute best-case scenario, I expect she'd come out of it looking pretty much like herself, only with horns, fangs, claws, and maybe a new skin tone. Plus she'd be anywhere from five to ten times stronger and tougher than she is now, most likely able to see spirits, and perhaps able to work some magic. Shapeshifting is very, very likely as well, though I can't say what particular forms she might be able to assume." Ambrose shrugs. "Of course, that's contingent on getting the seal's creator to agree to remove it, or having someone very skilled spend a lot of time studying the seal, and then getting every single step of the removal process exactly right. Any other method would involve a certain amount of stress - and pain."
"And if somebody were to, for example, just tear the whole thing off by main magical force?" you suggest.
"Considering that it's an Infernal seal? The shock would almost certainly kill her," Ambrose replies flatly. "And if by some small chance she survived, the sheer agony would likely drive her insane."
You wince, and are deeply relieved that you weren't seriously considering taking down the seal. Then you turn to the adults for advice. "So what do we do now?"
"I think you've done about all you really can do in this situation, Alexander," Masaki says.
"Not to mention more than you probably should have," Isshin mumbles. His wife shoots him a glare. "I'm just saying, Tatsuki's out cold, I just know that you're going to be having a shouting match with Akkiko before the day's out-"
"I do that at least twice a month anyway," Masaki returns.
"-yeah, but this time it's going to be for real! With a woman who's apparently part oni! Not to mention that I have to go yell at Kisuke for... well, all sorts of things."
"Dear, I have been yelling at Akkiko since before I met you. She has yet to do anything worse than yell back, throw the occasional punch, spill cheap sake on my best blouse, hit on my man..." Masaki pauses. "You know, the more I think about it, the more the idea that Akkiko has an oni ancestor makes sense."
"It explains why Tatsuki keeps kicking my butt in karate class, too," Ichigo agrees.
"You do realize that with the seal on, she's as human as everybody else, right?" you remind him.
"...let me have my dream."
You smirk briefly, then sigh. Both of the elder Kurosakis have a point: there really isn't much more you can do to help in this situation; and you've already done about as much damage as you can. Maybe it's time to bow out, and let the people who actually know Tatsuki take over?