I'd like to do the Blind Betrayal quest to help save Danse, but don't want to help the BoS.
What will happen to Danse if the BoS becomes my enemy? I take it the quest will be unavailable and Danse will be hostile?

Yes.

The BoS never finds out Danse is a Synth unless you bring them critical intell.

Personally, I joined the BOS as a mole.

But how do you roleplay helping rebuild Liberty Prime, as I'd get the feeling they would eventually use this or some other big weapon against me?

My Railroad PC is rebuilding Liberty Prime because he knows it's never going to be deployed and after the BoS is destroyed, he plans to seize it to guarantee the independence of the Commonwealth against all future retaliation.

That sounds good, thanks Because I'd really like to help Danse... I'm having a real hard time deciding on when to continue the main quest and thus making the BoS an enemy. What about Dr. Li, did you ask her to join the BoS or not? I guess it doesn't really matter since I plan on taking both the Institute and BoS down... But I'm curious

Edit: also, how do you feel about your companions responding on helping BoS or Institute? Ignore or just not bring them along?

Just remember Danse will no longer be willing to be your companion once you betray the BoS.

And I personally bring whatever companion was with me at the time, feels artificial to switch them because I know - in a meta sense - that they won't like something. Like, I brought Danse along to my first meeting with the Railroad, since my SS had no idea who they were or what they were up to. Once he learned more, as a loyal BoS knight he told Desdemona he wouldn't risk his life for a synth, but agreed to do a few things for Deacon, since Deacon seemed more reasonable and didn't hold his anti-synth sentiments against the SS. Even though Danse hated that. But, it was in-character for my pc, so I just had to deal with that bit of reputation loss.

1. I asked Doctor Li to join the BoS, albeit in my headcanon, the Sole Survivor gave her a message to hide in the basemant of the airport during the attack on the BOS.

2. Paladin Danse hates the Railraod while Nick hates anything related to the Institute. So I don't bring them along for operational security reasons.

Thanks for the replies! Will Paladin Danse still hate the Railroad once he finds out that he is a synth?

And it's a bit off-topic, but I'm curious if anyone has any roleplaying advice for (radiant) Institute quests?

Edit: I also don't like switching companions so I can still do a certain quest without their disproval.

1. I think so. He's a self-hating Synth and is kind of in a fugue state thereafter.

2. The Railroad actually gives you orders to earn Father's trust, "whatever it takes" so you could prepare for a Synth revolution.

I told Maxson off on the matter of Danse, and he simply "purged the records of the BoS of Danse, and then promoted me after it.
Danse remains alive and available as a companion which was what I had wanted.

Edit: I was in the wrong forum. My bad. On the phone.

Yes, but you must side with the BoS in the end to keep him that way. Also, don't parade him in front of any of your comrades in arms afterwards or things will get ugly.

So do you have to keep danse alive in order to get his perk or can you get it before you choose to kill him or let him live?

Yes, you have to spare Danse. That quest apparently counts as his companion quest. You don't get his max approval speech until after that quest.

Would be sad to arrive at Red Rocket and see that the other companions, and local defenses had scragged him simply because he still had BoS alliegence after Maxson kicking him in the teeth like that.