Make child div stretch across width of page
With current browsers (this question is dating a bit now), you can use the much simpler vw (viewport width) unit:
#help_panel {
margin-left: calc(50% - 50vw);
width: 100vw;
}
(usage data: http://caniuse.com/#feat=viewport-units)
From my tests, this should not break your flow while being easy to use.
Yes you can, set the position: relative for the container and position: absolute for the help_panel
Yes it can be done. You need to use
position:absolute;
left:0;
right:0;