Would Stolas make Blitzø immortal?
It doesn't really seem like Stolas has any strong connections or people he cares about outside of his intense attachment to Blitzø (and Octavia, obviously). He already proved he's willing to pull some massive strings and break universe rules just to let IMP access the mortal realm, first with the grimoire and now with the Asmodean crystal. With that track record, nothing feels out of the question when it comes to what he'd do for him.
We know imps are mortal and eventually die of old age, while Goetias are basically immortal. Once Stolas fully realizes that Blitzø's time is super limited compared to his, do you think he would try to find some loophole, spell, or deal to make Blitzø immortal too?
Personally, I think Blitzø would actually consider taking a deal like that, but under one strict condition: Loona has to get the exact same offer. There is zero chance Blitzø agrees to live forever if it means watching his daughter get old and die. He's way too fiercely protective of her to leave her behind.
As for Moxxie and Millie? Not so much. I feel like he loves them in his own messed up way, but at the end of the day, they're still just his employees. He'd probably just let M&M live out their normal mortal lives together since they're so codependent anyway.