I'm pretty sure Siren is correct here. Per Article III, Section 4 of the Constitution, senatorial elections are regulated and administered by the Regions, which suggests that federal activity requirements would not apply. However, since TDAS and Alex are invalid due to citizenship requirements (neither is registered to vote in the South), removing the activity requirement does not effect the outcome of the election.
Unless the number of posts is not part of voter registration rules, regions need to follow federal law. The new constitution still has a federal supremacy clause so I think regions have to respect the standard for the number of posts.
There was a court case under the old constitution though that affirmed regions needed to respect federal registration regulations.
https://uselectionatlas.org/FORUM/index.php?topic=171139.msg3691620#msg3691620
