last panel

Actually, seme was given as aphrodisiac by the lil weird dude (i think he is apart of the royal family or smt) in chapter one the night they got together. i'm not entirely sure but the drug also affects those around them, so it's like they both were affected/drugged...this was stated in an update somewhere, can't remember. Also, Trolese had kept his feelings for Mu hidden for about a year or so before this happened so my guess is that he didn't plan on making a confession but they ended up sleeping together and Trolese probably saw that as an opportunity for him.
Still i'm just a reader so interpret how you please ╮(￣▽￣)╭

It was involuntary on both sides, as @Connie of Paradise said, Trolesse was drugged that night. So he wasn't in control of his actions. But he had feelings for Mu and his brother knew about it. In order to stop them from getting together, as Trolesse would've ended up with no natural legal heirs to the Aslan family given that Mu was a common man, the brother sacked Mu and gave him compensation while making Trolesse forget about Mu.

I thought Trolesse was aware that it was an aphrodisiac beforehand? That's why I assumed it this way. I guess, I'll have a re-reading if I interpreted it wrong haha

Chapter 1 is where it happened. I think Mu was also slipped in a small quantity of the drug.

Somehow, I think Chapter 2 explained that Trolesse was already aware of the wine's content so he took it as an opportunity to finally got Mu. He mentioned in his bubble the name of the aphrodisiac so I believe he knows what it can do so still proceeding into drinking it isn't entirely an involuntary action to his side. Although, I don't see him as that bad of a person but the previous action was still wrong.

I thought that that speech/thought bubble belonged to the dude who gave Trolese the drug, not really sure though.

Yes, upon realising the wine's content, which makes him more accountable.