In terms of pedaling, it doesn't matter. The same ratio between teeth in the front divided by teeth in the back will feel exactly the same. The only rule here is that you are aiming to keep the chain in a somewhat straight line. Supposing you have two chainrings in the front: Never go small-small or large-large, as that would put strain on the components. For a 2x11, for example, usually all gears are safe except for 1-11 or 2-1, if that makes sense.

Since you don't want to shift the front gear while under pressure I try to shift it first before a big hill. But planning ahead is the only reason to pick one or the other of your overlapping combinations.

Here's a tool to visualize gear ratios that I found interesting https://mike-sherman.github.io/shift/

Cadence is also a personal preference, lower cadence is more based on muscle power (50-70 rpm) higher (90 rpm+) put more load on your cardiovascular system. In time you'll drift towards what feels natural for you Going up hill try experimenting with the cadence. You'll quickly discover what cadence you feel is efficient for you

In regards to your gearing question, it is hard to tell, it's about ratios between the number of teeth on the cassette and chainring. Find an rpm you like, adjust gear until you find that rpm

I wouldn't think of it that way. Just try to avoid twisting the chain too much. If your chain is all the way to the left in the front, keep it on the left half in the back. If you feel like moving the rear to the right half, just shift the front gear to the middle instead. Beyond that, just do whatever feels right.

I usually set the front to the ballpark I expect to be in for the foreseeable future, and fine tune in the rear as I go. It's usually harder to shift the front while pedaling hard uphill in my experience.