Is it a bad idea to buy plane tickets before the conference schedule is announced?
First, I would ask whether you think you really should skip part of the conference---you're going such a long way, and presumably it's a community that you care about, so are you sure you want to abandon part of it?

If you really do want to skip, though, I would recommend a third path that I have often seen work: contact the conference program chairs, let them know your travel dilemma, and ask them if they can schedule you on the prior day.  If they are likely to be flexible, this will let them be flexible now and let you book; if not, you'll know they're unlikely to be flexible at other times either and thus know that it's not safe to plan to skip until the schedule is announced.