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To give the short answer to (1) and (2): many solar controllers sense the battery voltage to decide whether they're facing a 12 volt bank or a 24 volt bank ... having that information *before* handling the panels' input avoids mistakes (since even an "18v" panel can have 22v on a sunny day without a load, which might entice the controller to call your batteries "24v" and try to charge them up to that level).
Some controllers have overriding switches, or use internal Flash memory to retain their voltage setting, but if you don't have that style (or even if you do, but leave it to "guess") you might get into trouble.
The controller *expects* to have its logic powered by the battery ... so it's nicely woken up and thinking before the panels appear.
On my El Cheapo (how do you say that in Chinese?) PWM controller, it's the same ... i have a fuse panel there with a fuse for all three connections (panel, battery, loads), so it's easy to pull and replace them in the proper order before maintenance. I also throw a blanket over the panels first (just 'cuz) when i'm planning such as assault.
--dick
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