1. General experience is that you can check your luggage in for LHR-SFO without having checked it in at ZRH. Recent posts by a user claiming to work at LHR check-in say this is being stopped, but there has not been any independent verification. This should be irrespective of whether you have an overnight transfer at LHR. What you probably cannot do is check in baggage for LHR-SFO before flying ZRH-LHR. If you have no convenient family member you can also store luggage at LHR (excess baggage company on the arrivals floor of T5) for around £8.50 per piece per 24 hours or part thereof.

2. Two separate questions there. Will you be allowed to short-check your baggage to LHR only? Possibly, but possibly not. They have a lot of people in the US who try tricks like this to access cheaper fares by travelling further, and some agents will be quite insistent that your baggage is checked to your final destination. And will you be allowed to take your LHR-ZRH flight at a later date? Depends on the fare rules for the fare you book. You haven't mentioned which class of travel you're planning to use, but in I class (discount Club World) it is almost always the case that no stopovers are permitted on the fare component, meaning your LHR-ZRH must take place within 24 hours of the arrival of your SFO-LHR.