If I might presume to answer #3 (and apologize in advance if I get it wrong)

Alternate tunings don't change the KEY....but what they often do is set a different set of default embellishments that make playing along with someone a chore if you play with more standard embellishments.

If you play "Red Haired Boy" in DADGAD your key of D has a natural default (sus 4?)that clashes with the embellishments one usually associates with the tune in its more widely accepted Key of A......EVEN IF you are capoed up to "A" pitch.

IT ain't the key, its the embellishments.