Just as "arms akimbo" has a very specific meaning, is there an "arms __" that specifically means what I described in the title?
I don't know of a single word that describes this. When I read your question, my first thought was the line from Pearl Jam's Jeremy:
arms raised in a V
Among friends, I jokingly refer to this pose as sticking the landing (as a gymnast might). YMCA interruptus is sillier and not as clear, but remains an amusing option.
Not one word, but I'd say you hit the nail squarely on the head in the question when you said arms overhead in a Y-shape.
I instantly knew exactly what you meant.
as a flight deck hand signal it means/called "fold" or "spread wings"
People often use "raised (as if) in supplication" to describe that position, which can go from a "Y" shape to straight overhead.
I don't know if there's a precise word for it, though. I'm curious to see if there is.