There is no need to block the mouthpieces unless you are storing them where something could crawl in. It would be better for air to get to them to keep any moisture that could accumulate from turning to mold or mildew. As to the bc yes, a bit of air in it to keep the bladder from sticking to itself if there is any saltwater that did not get rinsed. I have heard of people also adding a bit of mouthwash to some water and sloshing it around and then draining. This is presumably to get rid of any latent bacteria. As to the computer battery it may be better to leave it in. What does the manual say? Sometimes electronic devices need a small amount of power in storage to hold some basic functions. Unless it's going be 6 months to year( If that long I'd just give up diving!) the worst is that when you fire it up again it needs a new battery. Unless you are worried about the battery leaking. Not that big a deal these days with the new battery technology.
But the best thing is unless you are going to the military or prison and will be unable to dive, don't store it-DIVE IT! You are in California for god's sake. My gear would be lucky to ever dry out if I lived there! Hell it barely gets dry here in Pa. Even if it means freezing air temps(diving this weekend and they are calling for snow) or now that I can, under the ice. DIVE young man, DIVE!