Does this properly overlay tooltips and popovers and navbars from content below the modal backdrop?

Better than before.

In variables.less is defined:

// Z-index master list
// -------------------------
// Used for a bird's eye view of components dependent on the z-axis
// Try to avoid customizing these :)

@zindex-navbar: 1000;
@zindex-dropdown: 1000;
@zindex-popover: 1010;
@zindex-tooltip: 1030;
@zindex-navbar-fixed: 1030;
@zindex-modal-background: 1040;
@zindex-modal: 1050;

But then, in the modals.less

• .modal gets z-index: @zindex-modal-background;
• .modal-dialog gets z-index: (@zindex-modal-background + 10);
• .modal-backdrop gets z-index: (@zindex-modal-background - 10);

How you see, in the old code, @zindex-modal was never used!
.modal-backdrop gets same value like @zindex-navbar-fixed. So, that only works without error, because navbar is mostly in the DOM at top and modals are defined later in the DOM and so they shown above.

Groovy, thanks for doing the research and cleanup <3.