Your first suggestion could still happen but on a different day during lunch or break and Julius doesn't find out until later. Maybe Ross or someone witnesses the whole thing and tells Julius after everything is over.

We could do the affair thing during the party. Yeah, that could work. Later that same night, Jakob could ignore Julius leading to him becoming angry and sexually harassing him in a desperate attempt to show him his feelings? Maybe Jakob could somehow escape. This is all while he is still drunk. The next day, Julius can't remember much about what happened and when Jakob reminds him, he feels very guilty and tries to make it up to him but Jakob won't have any of it.