I haven't played Summoner route with different choices yet, but I think Therium is sort of like a bonus thing?
SPOILER BELOW!!
I don't know if this happens if you accept all of Therium's advances, since I turned him down at times. But at the end of the currently available Summoner route, he turns against you, causing a certain thing to happen which unlocks a new option earlier in the day. I THINK. Which then would be fully Scribbles route?