The PVSA is the "antiquated" law you are referring to. It states that a foreign flagged ship may not transport passengers from one US port to a different US port without a stop in a distant foreign port. A distant foreign port is described as any port NOT in North America, Central America, the Bermuda Islands, or the West Indies (including the Bahama Islands, but not including the Leeward Islands of the Netherlands Antilles, i.e., Aruba, Bonaire, and Curacao)
The law views back to back cruises as one cruise. The cruises you want to take would transport you from Galveston to Port Canaveral with no distant foreign port visited. So is, therefore, illegal.
Cruises from Puerto Rico are exempt from the law, so a back to back from San Juan to Port Canaveral doesn't have the distant foreign port requirement. And the B2B San Juan round trip with San Juan/Port Canaveral is legal.