you've absolutely hit the nail on the head - the trauma of parental death + being forced to grow up so early/take that parental role informs SO MUCH about the personalities and issues within in 3zun, and I think that's why I've not really explored it much even in the Happy AU.
...that said, @guqin-and-flute and I have discussed a sort of "AU of the AU" situation for the A-Fu 'verse where Meng Shi survives. she helped Meng Yao hide Xichen during the war (a whole kind of hilarious concept of its own tbh), and once JGS is well and truly out of the picture, after some strategic prodding from Yanli, Zixuan buys out her contract. ideally she would have been brought to live in Koi Tower, but the danger to her from Zixuan's mother just isn't worth it, so JGY sets her up all nice and comfortable with a little house and a good doctor and a healthy allowance nearby in Lanling city.
so in this AU yeah she gets to be a grandma and meet A-Fu!!! and it's fuckin lovely because JGY has told A-Fu quite a few bits and pieces about his zu-mu whiiiiiiilst also maybe possibly implying to him that she is dead, because A-Fu just could not possibly understand what a complicated clusterfuck of risk (in JGY's mind, at least) that relationship could wind up being, for all three of them, so it's best to avoid it completely. and then JGS is gone and Meng Shi is set up as an Educated Lady in Lanling and JGY is learning how to live without being terrified at almost every waking moment, and he realises that, OH, this is maybe a relationship that could be safely allowed now???
it's love at first sight for both of them. Meng Shi just ADORES A-Fu and A-Fu just ADORES Meng Shi and JGY is trying so fucking hard to not just burst into tears because finally someone loves his mum as much as he does!!!!! you know Meng Shi would be such an indulgent grandma when it comes to time and attention (lord knows she has spent her life listening to some of the world's most boring and awful men, she would have just infinite patience for her darling grandson), and I believe canonically she is crazy accomplished, so she definitely does a bunch of singing and music and painting with him.