The best way to mount a rear rack
There is another option that is preferable to either of the ones you mention: P-clamps. These are small strips of sheetmetal bent into a P shape and covered with rubber. They fit around the seatstays to create mounting points. You'll be able to attach both of the top rack arms that way. This will be much more secure. P-clamps are cheap and you can probably find them at any decent bike shop or online store.

Once you've got P-clamps on, I would mount the top rack arms as pictured in B1, more or less, but that's mostly a cosmetic thing. B2 wouldn't really be different in terms of load transmission.