Mrs Johnson and Lou Barlow have various aspects to their character. Sometimes they are happy - sometimes they are a happy Mrs Johnson and a happy Lou Barlow. Sometimes they are thoughtful - sometimes they are a thoughtful Mrs Johnson and a thoughtful Lou Barlow.
If you bear in mind that "a/an [noun]" gives the meaning of a/an/one example or instance of a [noun] - a cat = one example of a cat - and "a/an [proper noun]" will mean "a/an/one example or instance of [proper noun]" - an example of the Eiffel Tower - you will see that "a/an [adjective] + [proper noun] gives
A nervous America awaited the results = "an example or instance of a nervous America...," which we can restate idiomatically as "An example of America when it was nervous ..."
"blablabla" says a breathless Mrs Johnson. = "blablabla" says and example of Mrs Johnson when she was breathless.
"Dinosaur Jr. set to release new album mid-2016, says a nervous Lou Barlow" = "Dinosaur Jr. set to release new album mid-2016, says an example of Lou Barlow when he was nervous."
Can I understand these sentences, as if they were:
Mrs Johnson says breathlessly "blablabla"
Lou Barlow says nervously "Dinosaur Jr. set to release new album mid-2016"
No. There is no adverbial function in breathless or nervous - there is no information about how anything was said. There is only information about the subject's state at the time of saying it:
""I will kill the dragon" said an unhappy Sir Greybeard in a timid voice." Sir Greybeard is unhappy but speaks timidly, not sadly.
"I will kill the dragon said Sir Greybeard bravely, although he was a coward and intended to run away." He said it bravely but he was not brave.