Kirk likely didn't think to contact Sulu because he had no idea that Sulu had been in touch with the Enterprise while he and McCoy were imprisoned. He likely hadn't time to check the logs; things were moving pretty fast.
As for Valeris being so knowledgable -- perhaps she wasn't. Perhaps there were many other conspirators whose names were never revealed. What's-his-name the Romulan (played by Malachi Throne when Spock crossed over to TNG) may have had an idea of it, and he was at Khitomer (according to TNG). Then there was the crew of Chang's ship, too; Valeris never named them. So maybe Valeris was just in the highly-placed Federation/Starfleet group.