Can you post a picture of the situation? I'm not understanding the question from the description.

If the trees are going up end up inside the bed with soil piled around them that is a no no. The bed needs to stop at the tree and start again the other side.

I don't understand the question either. If you can't use something as small as bricks, I'm not sure what you could use. Pics of the space would surely help.

Frankly, in that tiny space, you could find a large container/pot that would work. Probably be cheaper.

Yes. I'd put a large pot between the two trees. You could make boxes each side or just use more pots. You must not pile any soil against the trees. One of them already looks to be planted a bit too deep. Can't see the base of the other.

Confusing. The last picture you posted of the brick box full of plants - is that what you built or AN EXAMPLE of what you WANT TO build?

The second picture you posted with the hammock in view - I assume that’s your yard and you want to build a box in that dirt void along the white fence? How wide is that space, measured from the fence out to the bricked floor surface? Less than 12”? Way too tiny for a bed.