Yes. It's informal, regional usage. It's somewhat rare, don't try to use it yourself. "Plumb" is related to the Latin word for lead, the soft, heavy metal ("plomo"). Some other words in English are derived from it. A "plumber" fixeds water pipes, which once were made of lead (so "plomero" in Spanish). "Plumb scary" probably refers to a "plumb line" (plomada). A plumb line is a tool used by builders to make sure that buildings are exactly vertical. It consists of a lead weight at the end of thin, strong cord. There are a number of words in English that carry the idea of "right" in the sense of perfectly aligned--physically or morally. "Right," "square," "true," "correct," and others. Rarely, "plumb" can be used in this way. A building that is perfectly "plumb" is one whose walls are perfectly vertical. "In erecting the walls, the logs should be kept even or plumb on the inside faces if it is desired to finish the interior with wallboard or plaster." So, by extension, "plumb" can mean "exactly right, as perfect as if you had used a plumb line." "Plumb scary" means she's not just a little scary; she's exactly scary, she's precisely scary, she's as perfectly scary as a plumb line is vertical. I am most familiar with this use of "plumb" in the phrase "plumb loco," a slang regionalism meaning "completely crazy."

Yes, but only in certain dialects. In this case, it's an Old West cowboy dialect. You might also hear this in some old Southern or Appalachian dialects, but it would sound strange to most people in other places.