MC is a SUPER in love with Alexei, and he is aware of this. Tiara gets so worked up around Alex that she can't breathe and sends herself into something like a panic attack. The "poison" is actually a tranquilizer he made to allow her to relax and be able to have s*x without working herself into a frenzy - hence the addition of something to prevent heart attacks, Alex waiting until it took effect to even kiss her, constantly reminding her to breathe, etc. Alex is actually one of the better ML's on consent - he is constantly checking in with Tiara, asking before he does anything to her, telling her what he's going to be doing and why. He's actually really sweet, and Alex's behavior later makes it clear that is is just as in love with Tiara as she is with him. I don't think he ever bought the story of Tiara cheating on him in the first place since it seems like he knows her very well and also realizes she does some dumb things because of her obsession with him, although there are a few lines right at the end that make me wonder if TIARA is actually the one who's the (scary) yandere -

"I couldn’t possibly turn Alex into a mu*derer.

Although, of course, even if the worst were to happen, I’d make sure that it wouldn’t be traced back to him—

—Because it was Alex.

However, my grandmother and mother advised me not to do anything rash."

- to me, this indicates that at some point Tiara HAS "done something rash" and possibly eliminated a rival in a very.... terminal way or similar. That is another frustrating example of where the author tried to be subtle and because the rest of the work is so obvious it gets glossed over and missed. UUUGGGGGHHHHH

MC doesn't want to die too horribly, meekly drinks, what she think is poison but was aphrodisiac, basically drugged up and pushed down.

She didn't dislike it, was more or less delirious and drunk in 'this is better than dying and feels rather pleasant cause duh, I'm drugged up with h**ny juice' mood.

As to whether she's later on pacifying a scary yandere by submitting to him (possibly Stockholm syndrome) or actually love him, we will never know, but it annoyed me to no end she never clarified that she never cheated on him.