You are probably right with the tournament and Billy's thing happening at the same time, although that would mean no Jeremy in the Billy thing.
Well, it can be "roughly" at the same time, meaning there's a chance for Jeremy to attend both. E.g. if the game follows its usual structure, then we'd have:
Ian's side: beach trip
Lena's side: beach trip (or other content, if she doesn't attend)
Ian's side: tournament (and perhaps some Cindy drama, she'll be back from her photo shoot)
Lena's side: Billy's event
the last two can overleap, but it's also possible for the tournament to finish before the photo shoot ends, and for Jeremy to show up at Billy's event afterwards.
It could be also compressed somewhat, i.e Ian's side: beach trip + tournament followed by Lena's side: beach trip/alternative + Billy's event. If it was all fit in one chapter.
Incidentally, with things happening in this order it'd give chance for "npc" Lena to pitch the Holly idea to Ian, based on her game flags, and player as Ian deciding whether to go along with it.