First of all, let me say I am glad you pushed me out of my comfort zone, which is not something I take to easily and something I don't tend to do on my own. I know we don't have to follow the Additional Likes session, but I try hard to, because, I think, of the challenge, on top of the fact it's just nice. The talk about politics and misogyny and such had me absolutely terrified. I finally came up with this, but it was almost late. I talked to different people about the prompt, finally I came up with Regulus Black as a prime character. I defintely don't regret trying to come up with this. It wasn't perfect, considering I've seen much better fics that involved politics, but it's something I don't even think about.
Actually, because I don't think about it, Regulus was a good character to do this with, the way I see him. This Regulus is a bit of me, especially me at age 18 having just graduated. He's naive, even a bit stupid, and blindly thinks everything will be okay and he can coast through life as long as he has the love of his parents. But something you do is usually, at some point, going to scare or anger your parents in a way you never expected, and throw you off-balance, like your comment about the party lines shifting I think.
I enjoyed the idea of something sparking Regulus to look at the things he was doing and think hard about them. I don't reckon it was easy to do either.
I'm glad you noticed Bellatrix's emphasis on tradion and her original desire expressed here to have sons and to be the perfect hostess. There's something about the way she talks to Narcissa about sons in Half-Blood Prince that I think points to a real bitterness, and before Azkaban having an heir was probably a top priority. I wonder if she got any flack from the other Lestranges if any were still around.
As for what Regulus and Rabastan's relationship would have been like, I think it would have been healthy and beautiful. I think Regulus's growth would just add to their connection, as long as they truly did appreciate each other and love each other. However, I can't foresee them going undetected forever, because the two are used to being, for the most part, upfront about themselves. Hiding something as big as a relationship would be torture for Rabastan, but at least he would have recognized Regulus's needs first and kept quiet unless he slipped on accident. Rabastan and Regulus don't think quite as much as, say, Rodolphus or Lucius.
I think Regulus also might have had to confront his parents if he fell in love with Rabastan, because I don't think he would have been able to break things off, even in the event he found a suitable woman, because he would feel he owed Rabastan, plus he would love the man. And then he'd be going through this all over again, but he'd have somewhere to go at least. The two of them might run away. And Regulus would have shattered his loyalty to his family. But someone would have done it before him (Sirius) and he'd want to stay loyal to the person who didn't expect him to do anything except live the most fulfilling life he could being the best person he could be (Rabastan).
I would hate to see Regulus die while they were still together. I think it would really crush Rabastan to see him die and not be able to explain why he was taking it as hard as he would be. And then maybe he would have spilled and Regulus would have gotten burned off the tapestry in the shock produced. At least when it came from Regulus, his spot was secured.
Regulus's parents came through for him by not taking him from the tapestry for a mistake, but the shift in their relationship definitely scraed Regulus who was not used to upsetting them. Therefore I think if he was forced to choose between the two, he might stick with Rabastan if they had had a long relationship. He didn't really get a choice in the fic, which is unfortunate, but I am glad they were not together when Regulus died, and also glad Rabastan never found out the treachery Regulus had planned, just in case he was more loyal to Voldemort than Regulus, which in his shock he might have been.