Wilmington has quite a few evacuation routes...Though I-40 is the only one with reversible lanes.
The completely 4-lane and continually upgrading of US 74 is very feasible...and you all are making Wilmington out to be a larger city than it is. The metro area is only 350,000 people. The main reason for this proposal is to attract more ship traffic into the Port of Wilmington. If there is "a direct (one number route) interstate connection" from Wilmington to Atlanta, they hope it will attract more vessels into the port. (That's how I-40 was eventually moved to end at I-95 in Smithfield in the early 70s to Wilmington at the end of the decade, and also why US 117 was extended to end at the port earlier this decade.)
This is also why there is a group trying to promote making US 70 a freeway in its entirety from Raleigh to Morehead City to attract more business to the Port of Morehead City. (which in the 70s there was debate on where to extend I-40 to Morehead or Wilmington).
Plus, I-20 would not hit I-40 unless it was moved onto I-140.
Personally my choice, as I said in seroads, would be extend I-20 northwards along I-95 to around Exit 170 (SC 327) then have it run to meet I-73 between Marion and Mullins.
I do think that in the end this will end up with I-74 running to Wilmington vs. North Myrtle Beach, and I-20 not budging at all. SC is putting it's entire resources into I-73 - the overall impact of that route is greater in South Carolina's mind than extending I-20. When I-73 is finished, then yes I think South Carolina will try to extend I-20 to at least I-73.