I sincerely doubt he's lying. There are many things that Bill does not know in regards to living as a human. I have had to remind him no less than three times that he needs to sleep. And he's been living off of uncooked food out of the fridge and snacks from the general store this whole time. Even if it were a ruse, it seems excessive to wait five weeks before putting it to use.

[That is like...an excessively long con. Who would even...]

I see no reason to rule out Clover or Lucy yet, however. Even if they were together after 7:15, they are both suspects. There's nothing to say one or both of them aren't lying or that one of them didn't manage to slip away for a brief period without the other one noticing.