Short answers, pulled from the story.
A plesiosaur is an extinct Mesozoic marine reptile belonging to the order Plesiosauria, part of the Sauropterygia. Plesiosaurs had a broad flat body, a short tail, and four long flippers, breathed air, bore live young, and may have been warm-blooded.
Plesiosaurs first appeared in the latest Triassic, possibly in the Rhaetian stage about 203 million years ago, and became especially common during the Jurassic. They all went extinct in the Cretaceous-Paleogene extinction event at the end of the Cretaceous, about 66 million years ago.
No, plesiosaurs are not dinosaurs. Like the ichthyosaurs and mosasaurs, the genera in Plesiosauria are not part of the clade Dinosauria, and depicting them as dinosaurs is a common misconception.
The genus Plesiosaurus was named in 1821 by William Conybeare and Henry Thomas De la Beche from a partial skeleton in the collection of Colonel Thomas James Birch. The name comes from the Greek plesios, closer to, and a Latinised saurus, marking it as nearer to the Sauria than the fish-like Ichthyosaurus.
In 1867 Edward Drinker Cope reconstructed an Elasmosaurus platyurus skeleton with its vertebral column reversed, placing the skull on the tail. Othniel Charles Marsh pointed out the error and Joseph Leidy proved it, and Marsh later said the humiliating affair was the cause of his lifelong rivalry with Cope.
No, the scientific community rejects the idea that the Loch Ness monster is a surviving plesiosaur. The neck could not lift the head out of the water like a swan, the loch is too small and food-poor to support a breeding colony, it formed only about 10,000 years ago, and the youngest plesiosaur fossils are over 66 million years old.