Just noticed that I'd failed to answer this:
>Was Corwin adopted into the water tribe? I seem to remember reading
>that somewhere, but I advance in years and my memory isn't my strong
Not formally, but his was a fairly familiar presence around Senna and parts of Nanisivik (the Southern Water Tribe capital city) as a boy; before the Great Interruption he was usually there for a week or two (not contiguous) per year, long enough and often enough that Korra's neighbors got to know him. That's where he knows Karana from (as seen in the changing room scene in Past Is Prologue).
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Ceterum censeo Carthaginem esse delendam.