I think him finding out would be really interesting, or at least - has the potential to be if handled right ...I think his calmness would have disappeared if he had ever found out about the AR. Perhaps it is better that Angel doesn't know.
But to be clear - because it was a long post and I don't blame you for skimming - but I'm writing season 6 of Angel, not Buffy - so this would be Angel finding out about it years after the fact and he would be the lead.Such a bug for me, I was never a fan of that character while begrudgingly admitting the characters arc and development arc is coherent and complex. Feels like the short handed the lead character for a supporting character.
Totally, I expect Angel to understand Spike's soulless worst actions as he committed atrocities like him & is atoning for them. Angelus from BTVS S2 hurt & traumatized Buffy badly too. I expect Angel to give a chance to Spike.I think it would have been very hard to write him discovering this and have him getting angry without making him look like an insufferable chauvinistic ass - especially if Buffy was in the scene. After all, Spike didn't have his soul then and if anyone should understand the difference between the demon and the man - it's Angel. And if anyone has no leg to stand on when it comes to judging or forgiving the man for what the demon did ... it's Angel.
Xander's anger in the moment, finding her in the bathroom, is understandable. His telling Dawn about it days later is unforgivable. In one, he is reacting to the visible sign of his friend being hurt, in the other he is overriding his friend's wishes and privacy to spread around his anger - showing he actually cares more about his anger than he does Buffy's hurt.
So with Angel, so long after the fact and with Buffy clearly moving on and even choosing to keep Spike in her life, where does the anger come from? That a soulless monster hurt her? Well ... Angel's been there and done that. He's perfectly happy for Buffy to forgive him. Expecting one rule for him and a different one for Spike would make him an ass.
but Angel also famously blames himself entirely for his past actions - it’s why he feels so much guilt about it. I think Angel would try to kill Spike if he found out about it, its not some faceless girl that melts into the rest of their atrocities it’s Buffy! I think that’s why they never bring it up with so many characters because Angel and Giles would both try to kill him (I actually have always thought that was the angle they should have went for with why Giles tried to have Spike killed in LMPTM) actually I’d add Faith to that as for for people who would try to kill Spike in s7Totally, I expect Angel to understand Spike's soulless worst actions as he committed atrocities like him & is atoning for them. Angelus from BTVS S2 hurt & traumatized Buffy badly too. I expect Angel to give a chance to Spike.
Now about Angel's reaction to Buffy & Spike, I don't think he got a believable or strong reaction as he was really calm about it. In the past, Soulless Spike mocked Bangel quite often and was put off by Angel having a soul. The last time Angel saw Spike, it was from In The Dark & Spike hated Buffy back then.
I would expect from Angel a stronger reaction to the idea of Spuffy (especially with soulless Spike) & of Spike falling in love/having an obsession for Buffy considering how Soulless Spike treated Bangel back then.
But he has no right to do this. Buffy has forgiven Spike - if Angel tried to kill Spike after the fact for something Buffy had forgiven then that would be done out of a form of possessive rage over Buffy and completely deny her her agency or right to make her own choices. It would not be done to protect or help her, it would be done to make himself feel better - it would be a vengeance he had no right to. Buffy tells him Spike in her heart, he would be killing someone she loves - which would cause her the kind of pain he is apparently so angry about Spike causing her - just to satisfy his own need to get retribution or make himself feel like 'he did something' (even though nothing needed doing). It would make him an insufferable arse worse than the very worst of Xander.. I think Angel would try to kill Spike if he found out about it