Well, I can't see how a pipeline from Oklahoma to the Gulf Coast goes anywhere near Nebraska, existing pipeline or not. Without the "missing link" from the Canadian border to Cushing, I don't see how the Cushing-Houston tie does anything.
They are in effect breaking it up into several different projects. Once complete, you'll have a continuous pipeline all the way. If they do it that way, they get it built. Some of the other pieces are already being built or about to be built.
With North Dakota's substantial contribution (now ranked third in monthly production), the pipelines just can't carry enough oil, so there are unit trains and even haul trucks moving crude oil.
The 'discount' spoken of means oil production companies are getting paid as much as $30 less than spot for the oil (usually at par with WTI) at the wellhead because of transportation costs/issues.
Expanding the pipeline capacity by running additional lines alongside the existing ones does two things: Increases capacity, and dodges the EPA bullet by utilizing routes which have already been approved. Those lines will be north of Cushing, OK.