... how does one place a new container or item IN-BETWEEN two existing ones?
I find the easiest way is to use the Element Tree.
If the new element is to go immediately after a non-container element (referred to below as X), I use the keyboard up or down arrows to select X, and then in the Content panel click on the element to be added.
If the new element is to go immediately after a container element, I do the same as in the paragraph above, but then go back to the Element Tree and click on the newly-added element. It will probably be inside the container, and I simply do the following on the keyboard: ctrl+left_arrow then ctrl+down_arrow.
Until one has played around with the keyboard ctrl+arrow keys for moving up and down the Element Tree and for moving elements from parent level to child level, or from child level to parent level, the matter can indeed seem difficult.
It's particularly worth remembering that to make an element (X) a child of a container element, one positions X immediately above the container element, and presses ctrl+right_arrow. Pressing ctrl+left_arrow will be seen to reverse this.