Firstly, the infidelity issue. What has he actually done so far other than making Buaji basically his own personal love-muffin? He's expressed strong creepy vibes towards Khushi, and lusts for her like a meth addict for pills. Yes its awful and yes we all take showers after the show to get the residual creepy off of our eyeballs. But in reality, he's expressed these emotions to the audience and himself.
Not even Khushi can claim she's been made uncomfortable by Shyam, or that he said anything to her to indicate clearly his interest. This is not enough to condemn him, or get him removed from Raizada House. Assuming his unmasking is public, which I dont think will happen btw, his ability to spin the situation can make it seem like he's attracted to Khushi, but hide his obsession from the eyes of the world. ASR might think oh, he felt tempted, and made some mistakes in becoming a PG, but he hasnt done anything per se, and give him a chance to fix himself.
From Buaji's and Sashi babu we cant expect support. He has not said, in explicit clear words, that he is interested in a marriage (which he obviously is). He's letting Buaji speak for him without making her do it - its all implied. Where has he said "I will marry Khushi, someone fetch me the sherwani I usually wear for my extra marriages." Never, right? In that sense, he's safe. There is no way ASR can prove to his sister, or even to himself, that he's going to cheat on Anjali with an actual second wife. Wheres the infidelity? Right now, he has a weird living situation and has at most said a bunch of lies about not knowing Khushi.
Finally, hes a good husband. You dont think so, I dont think so, but his wife, the only person who needs to believe this lie does think so. And he makes her happy, fake as it is, and he delivers support to her when she needs it, hypocritical as that support might be. Bottom line, ASR protects his sister ferociously, and has given everything to make a happy world for her-he says his sister doesn't understand dunia-daari, and he works to make sure she's kept in this dream world.
Thus, unless there is a public reveal, and not an accidental private one, there is no way he'll tell Anjali something that will destroy her. How can he break this belief in her? I think he will choose to prevent Shyam from following up on the cheating plans, but not actually tell Anjali anything. He might not even say anything to Shyam, even after knowing about Khushi, and simply choose to take Khushi out of the picture, ansd make her his bride/girlfriend and opt for a silent war.
I think we'll end up with Khushi and Arnav in the house on one side, Shyam and Anjali (unknowing) on the other, and a war between the couples will be the outcome of Shyam's reveal.