Did Cersei know?

Cersei wasn't protecting Tommen by keeping him in his room, she was making sure he didn't get in her way. That's why Gregor left as soon as it was over. He was unable to stop it, so she didn't care what he did then.

Yeah she was surprisingly calm. I think she accepted that Tommen will die as soon as she saw Myrcella's coffin in the premiere

She already knew she had lost him one way or another in my opinion. If the Sept wasn't blown up and her enemies destroyed, she lost him to the Faith and the Tyrell's. I think she knew the risk of alienating him from her with her actions (although I don't think she exactly expected suicide) but was willing to accept that consequence, because he was already gone from her. I don't think she expected him to kill himself, but she wasn't overly affected because she had already come to terms with losing him (just in a less final way)

Cersei gave upon Tommen way back in episode 3 I think it was, but you can read her face in that scene where they embrace. She knows he'll die soon.

For me it's much more complicated. Season 5 was for her fight to take back her children. She send Jaime after Myrcella, and started to plotting in KL to tear out Tommen from "Tyrell bitch's" claws. First one end in failure about which she didn't know yet, second one end in half victory-half failure and for the price of humiliation. But at the end of S5 she still got enough strengh to survive this and fight more.

S6 start with her world collapsing. Myrcella is dead, prophecy is true. Cersei had a moment of breakdown and wasn't sure what to do next. She gathered the rest of her strengh to fight for Tommen. But day after day she realized that Tommen is disapointment and maybe isn't worth to fight for. First he didn't allow her to attend in Myrcella's funeral. That was a slap in her face. She walked naked across the city to be with him and he didn't appreciate this, didn't fight for her, did nothing at all. But he asked for forgiveness and she opened for him her hurt mother's heart. Tried to help him get his wife back. Then was a second slap in her face. Tommen made an alliance with Faith and her enemies High Sparrow and Margaery triumphed. She once more tried to get alliance with Olenna to what Cersei thought could help both Tommen and herself. But she failed and her son gave her last hit: a stabb in the back by banning trial by combat. They both knew what it maent for her.

Cersei realized that she had to focus on her own surviving. She decided to kill her enemies, even if that would hurt Tommen. Cersei put her war gown, her armor, took a glass of wine and made sure Tommen won't interrupt her in enjoying sweet revenge. It looks like she gave upon him, but I think it meant to be only temporary. Yes, she shows everyone, that she wants power for herself and has an intention to marginalized Tommen. Cersei was aware that she will hurt her son really hard, but I'm not sure if she was able to predict his suicide. IMO Cersei was tired of him and decide to focus on herself at this moment and think about him later. But later he was gone.

After Joffrey's death it was pure fury, after Myrcella- breakdown, after Tommen- void. Not that she felt nothing, when she saw his body all her emotions exploded and vanished in the next second. There was nothing left for her. A hole in place where the heart should be. She was able to do the wortst crimes and the greatest sacrifices to protect her children. After they're gone there is only herself left. And Cersei is now a mix of a void, madness and determination. She won't stop at nothing to make everyone suffer like she suffered, taste her vengance and die after. She wants to make whole world turn in ashes and doesn't even want to be Queen of those ashes. Just makes herself and everyone else vanish and left after only the desolation.