I don't believe cider's got that short of a shelf life- I've got stuff sitting around from back in '05 that is still fine and dandy. If the alcohol content is fairly low, then you might have an issue, but if it's in the neighborhood of 5% or higher, you shouldn't have a problem letting it sit around for more than a year.
The fact is, I've got a five gallon batch with two pounds of sugar and two pounds of honey that is still fermenting. I started that baby back in June, and right now it's still burping up a bubble a minute through the airlock.
The oxygen caps are a bit of an expense, though, if you're planning on drinking your stuff really fast. I just use the cheapie, normal caps and I have never had a spoiling problem.
Just use the full pack. Often times, when I'm doing five gallons, I have to throw in two or three packs of yeast to get things going. But that's just me.
The secondary fermentation is when that vigerous, violent fermentation at the beginning ends and the actual yeast reproduction goes on in earnest- that's when all the oxygen in the juice has been used up and the most alcohol is being produced. Do not bottle until the bubbles stop leaving the airlock! Bottle grenades are not fun. I should know.