If he had the skills to make Yammy into a bomb but no skills that would let him make the Sarcophagus, he would've made a Yammy bomb instead of the Sarcophagus. It would've been cruel, still, but less cruel. Giving Szayel more skill won't simply alter the form his evil takes shape into a different but equivalent one, it will give him a greater capacity for inflicting cruelty unto others.
I think you're somewhat overestimating (exaggerating?) the use and utility Szayel's going to get out of modifications made to the Sarcophagus, re: proliferation and shit. Skills tend to be kinda broad in how they're applied and encompass a variety of related ideas. And given his compliments and the way he talks about it it's clear that Szayel doesn't view La Maraña as meaningfully
different than what he does, he just thinks Nemo's dressing it up in different language. And I don't think he's really wrong? Not where it matters? Fundamentally we're not granting Szayel any capabilities he doesn't already have and we're not providing him tools to accomplish anything he can't
already. We're supplying another perspective, advice, and critique along with a second set of hands able to roughly keep up, a sort of parallel expertise. We're teaching him in the broadest sense, we're hardly passing along the secrets of our art.
They're going to suffer no matter what we do, and tainting how we view Maraña would impact our ability to keep trying to make Las Noches and Hueco Mundo less terrible places in which to exist. It would also give Szayelaporro additional tools with which to torture more people, while potentially betraying our mentor's trust that we would not pass on his Art to the blatantly unworthy. Given all this, I think there's a pretty strong utilitarian argument in favor of going full Rohan here.
I'm not a fan of the utilitarian argument tbh and it's not one that I'm particularly making. It's not really effective I find since it tends to skate by, y'know, a lot of the real reasons people do shit in favor of pointing at an equation and going "but do it tho".
I would argue that this
is a use of La Maraña to make Las Noches a less terrible place. It's not in a grand and sweeping way, it's honestly not even in a way that might matter to the person involved 'cause Yammy's pretty far gone but overall I think that it's a positive application for Nemo? Unpleasant yeah but ultimately beneficial in terms of growth and worldview. It's not a fantastic set of choices and it's not even necessarily about "oh she's just soothing her conscience" it's...I dunno how to properly articulate it. It's using something she has, whatever scraps of control she has over an otherwise unwieldly situation to try and make it
better. Like it's not morally damning if she leaves Yammy, like I said he's pretty far gone and it's not like he'll know. Her influence on this situation is small and her contributions minute compared to Aizen, Szayel, and Cirucci even. She's really not to blame for what happened to him even if she feels like she is.
It's more that...well Yammy's basically helpless? Which is a weird thing to say about a doom tank with a life expectancy measured in months, who's bound to go out as basically his own funeral pyre. But he has nobody who cares about him, nobody who misses him, and nobody who really mourns him. There's nobody else but us right now who could make anything even the slightest bit better for him (well able and
willing). Using our influence to ease his pain, help the person who needs it most even if that person is ultimately fucked is kinda...in the spirit -lawl- of what's changing in Las Noches? Making it better? "Even if it's pointless you still have to do it" to pull from Emil. Ultimately the arrancar are kinda fucked from the start, members of a doomed army in service to a God that doesn't care. But even though it's bleak and hopeless they're still managing to get by in their own way, even if it's just moment to moment or day to day. And that's really just this writ small y'know?
So yeah, to be a little less sappy about it I don't really view this as a perversion of the Art, as tainting it or corrupting it. A compromise yeah but Nemo's not strong, she's not an Espada and even the Espada don't get to make the rules really. Just the rules Aizen doesn't care to already set. Compromise is what the world's giving her.