No, Horus did not do it willingly. The mirror possessed Horus, and us not seeing Horus’ mind dialogue is another sign of him being possessed. You can see he’s not the same as before, the way he doesn’t talk much and continues to do such crude things. It wasn’t him by will.
Seth needs someone who will understand him and Horus is one of very few who understood his suffering and knew he was suffering.
And I believe the chapter was around 33-34/around 30 something.
It's actually an interesting question, whether Horus can still be what Seth needs after having raped him (willingly or not). If these were human beings, I'd say, no way in hell... as their limited lifespans would never enough to solve, together, such a complicated trauma (since willing or unwilling, a grave humiliation of one by the other still indeed took place... and even if they were unwilling, the best thing the unwilling "perpetrator" could do for their victim is probably to leave them the heck alone. BUT since these are eternal gods, who have the rest of eternity to work out their issues, then it's a definite YES! Because they DO have a long time to "get over it", and it's obvious no other person would ever come close to understanding Seth as well as Horus can!

Regarding that twitter theory at the end...mmmh, I kind of agree, because I ship Horus and Seth, BUT we need to remember that Horus too, at some point, kind of r*ped Seth (I'm not talking about the not-canon extras at the end of S1, I am talking about that moment between the second and third trial, before Sekhmet goes and start talking to Horus, who's naked on the ground; I don't remember the number of the chapter, sorry. Still, moments before that, Horsu was assaulting/fighting Seth, and that's also when Horus discovers Osiris' and Seth's past through Sekhmet) so....is Horus really what Seth needs??? Mmmh I don't, it kind of feel iffy to me.
Regarding that cover art with the hands, I think that Ra's hands are there because the author has been taking a lot from the original Egyptian myths and is adapting them to their own liking to shape Ennead's storylines.
According to the myths, Seth is one of the most important deities (together with Anubis, Bastet and Osiris) in the Duat, or more precisely, in the path that Ra (the Sun) takes every day in his/her boat, from dusk until dawn, before getting "reborn" again. In the Egyptian mythology, Seth is part of Ra's retinue. So, I am guessing that if Ennead's author took this bit of Seth's original lore from the myths, then of course Ra's hands are on him too in that cover, because (I'm guessing) in the future Ra is gonna have a more important role...kind of like Ra already had during the trial (although, I am curious about Ra's new child...she was very pregnant O_O)