Explanation??? (SPOILERS)

I thought the turtle was just lonely. She destroyed the boats to keep the man on the island, and transformed into a human to be with him. Turtles grow very old.

As for your last question, it sounds like you already found a pretty good answer. It's all up to interpretation, I think.

I have most of the same questions as you. One thing that came to me at the very ending, when the woman transforms back into a turtle, is that he might have made everything up in some sort of delusional/lonely state of mind. Note that both the young boy and the woman swim the same way as sea turtles (without use of legs)! I feel it's a very sad story about a man who subliminally creates a fictional life which he desires, plagued by both loneliness and deep rooted regret for killing the red turtle. Either way, I think it is all up to interpretation indeed.

Don't take it literally. Did it hit you emotionally? Take your cues from that.

I am not purporting to know the exact meaning of everything, but I am certain it's just allegory for life in general:

Lost at sea in the beginning:
Lost in life... For one reason or another a lot of us have plans and dreams, but we might still find ourselves in trouble or lost or have difficulties and meet disspointment while trying to get there. We might not even know where there is.

The Island:
A place (physical or otherwise) you end up at by circumstances that are seemingly arbitrary, but at the moment necessary for your survival. Not what you planned for and you fail to see any good coming out of it as all you yearn for is a way out.

The Turtle Attack:
I took the turtle foiling his feeble attempts to get off the island (that would most certainly lead to his demise) as a person caring enough for his well-being to intervene while knowing full well that it would be perceived as unwanted meddling.
We are not always clear headed when we have a goal, or we are in such a state that we are oblivious to the dangers of our actions. Having someone that cares enough to ground you even when risking you hating them for it is indeed lucky.

Turtle Woman:
Well first we have the rage and then the guilt and then comes understanding.
The man was oblivious to the beauty of this person that tried to help him steer clear of trouble and after he lashed out he felt remorse as he contemplated further, and then realised this person cared for him. She transformed from someone he didn't even notice to someone beautiful as he opened himself up to it.

Staying:
No half measures. She cast off her shell and devoted herself to him right here and now, and so did he by ditching the raft. You have to decide to commit. If you truly want happiness, you don't find it by constantly searching for it - you decide to make it happen. Home is where your heart is etc etc. If you allow yourself to the possibility that maybe your goals can still be reached in a different manner or that they have changed/matured and instead of blindly looking ahead you can assess everything around you - you just might find you have all you need.

The child:
Well that's part of life for a lot of people isn't it? It just drove it home that much more (life paths) that he was his own person and grew up wanting to find his own path and they had to let him go.

Tsunami:
Random life crises. Family sticks together.

Death:
Well unfortunately that's how it ends for all us.

Que tissues.

Woman to turtle:
You got me. I didn't feel it mattered much.

In conclusion:
I felt (because with emphasis on the felt) the movie was about a life. General enough to evoke feelings of recognition in most of us even though most of us have not been castaways or had children with turtles.

Perhaps it's just a straight up narrative.

Turtle/Woman was some kind of water god that wanted to trap the man on the island and sire a son with him.