Everyone has a different riding style. Personally I have bad knees so i have always been more of a "spinner" instead of hammering big gears.

What is more efficient? I personally believe that maintaining a decent cadence range is probably the most efficient. change gears according to how it affects your cadence. I usually range between 70-90 rpm unless i'm going up hills. I ride long distance so i usually need to really pace myself in order to be comfortable for the whole ride. Crank is for major changes, cassette is for minor.

I don't think that minimizing your cassette use to maintain a "straight chainline" makes much sense since chains are like 20$ and will last for a good long while even when using all of your gears.

as for your second question it would be a very small difference in gears if your crank was able to accept a 34 tooth chainring. I think you would get more of a benefit without much cost if you were to bump up your cassette to a 28 if your rear derailleur will take it.

I live in seattle and for my heavy cross check i have a 48/34 and a 12-28 cassette w/ a short cage 105 rear derailleur. That being said i'm really going to wish i had a triple when i load it up with camping gear.

Good Luck!