So...what is Sakura then here? I'm hoping the situation here with the Matou may, hopefully, be less awful than Fate canon?
Yes, but mostly by the miracle of low standards. It's more or less been transferred over (Honkverse has plenty of unethical mad scientists who appreciate having experimental subjects), but I didn't see a need to transfer over the sexual elements of the Matou dickworms, so I didn't.
With that said, while she would be all too happy to be a daughter of the Tohsaka again, Rin doesn't actually
know that and is under the impression (in large part because she wants it to be the case) that she's having a happy life where she's been adopted into the Matou.
So how much has her lifespan been cut down by?
She'll know when she reaches it. It's not easily predictable, the damage occurs in spikes when the honkai they're empowered with and fighting surrounded by overcomes their stigmata-enhanced resilience and inflicts lingering physical damage - the shortened lifespan is essentially the aggregate effect of all of those incidents adding up until the body just doesn't really work anymore.
Statistically, for a Valkyrie of Luvia's Honkai tolerance (high) and general temperament (bold, aggressive, risk-prone), her life expectancy is into about the mid-sixties, but this depends heavily on the situations she ends up in and how well she manages them.
For another example, Sakura has about the same tolerance but is timid and extremely cautious, so a Valkyrie with that profile has a life expectancy into her seventies.
Shirou, on the other hand, has the same profile as Major Murata - low tolerance, extreme risk factors due to temperament and a poor understanding of personal limits - and has a life expectancy of about thirty.
However, these statistics were assembled for conventional Schicksal Valkyries, who operate in a purely paramilitary environment. Luvia is under conventional Schicksal training and operations, so that's about her statistical expectation, but Shirou is at St Freya. St Freya is a much more protective environment with a much gentler approach, and expects to achieve higher life expectancies, but it simply hasn't been in operation long enough to gather any data on the topic - St Freya's oldest alumni are about twenty years old, the school's been in operation for like four years.
P.S: Will Saber possibly be here?
There is someone with a King Arthur stigmata, and one of the votes will meet them.
I'm 99% sure I know that character, and if you're doing what I think you're doing, I will laugh so hard.
Ah, a fellow of taste.