Scenario: Captain can't serve their penalty as a blocker

That is very clearly the intention. That does however not resolve the contradiction that they according to 1.2 can no longer be captain while serving that penalty. Thus technically the penalty is not served by the captain, but by a substitute.

It also opens the question at what moment the penalized team is entitled to designate a new captain. According to my interpretation they should be allowed to designate a new captain before the jam in which the former captain serves their seventh penalty.