There are two different meanings of the word legend in your question.
In the first:
A Hero Becomes Legend
The meaning is that the hero, is going to enter folklore, and enter into the popular cultural storytelling of that time.
- A traditional story sometimes popularly regarded as historical but not
authenticated. ‘the legend of King Arthur’ - OLD.
The second use of the word legend with a, has a different meaning and as you conjecture, refers to an individual person. Specifically it refers to the fame or notoriety of an individual person.
- An extremely famous or notorious person, especially in a particular field. ‘the man was a living legend’ ‘a screen legend’ - OLD.
So to your questions...
I think if "legend" here meant "person" that it should be countable
and have "a" like "A Hero Becomes A Legend".
Legend can refer to a person in both cases, the difference is whether we are talking about that person entering into popular folklore (first definition above), or about the fame or notoriety of that individual (second definition).
But since it doesn't have "a", I think that "legend" here means a
"story" as in "Legend has it that...".
Yes exactly correct, as per the first definition above.
Also, is it possible to use the word as a countable noun meaning "a
person" in this title and in the clip and say "A Hero Becomes A
Yes you can use legend with a countable noun, as per the second definition above. However it would not make much sense in the example you offer, because:
A hero is:
A person who is admired for their courage, outstanding achievements,
or noble qualities. ‘a war hero’ - OLD.
A hero implies popular consent. A hero is someone who is already admired for their courage, outstanding achievement, or noble qualities, and would therefore already be famous.
However you might say something like:
My hero becomes a legend
Because if the person was just a hero to you, they could conceivably still become a legend (famous - see definition 2 above) more widely with other people of their time.