Snape couldn’t have stopped Harry from going to the Department of Mysteries

In Book 4 Snape had only just managed to convince Voldemort of his loyalty through words alone. In Book 5, as the only Order member staying at Hogwarts, the spy was supposed to help Voldemort's plan succeed instead of ruining it. If Harry had been truly obstructed by Umbridge, Snape might even have had to both send words to the Order and find a way to secretly help Harry head to the Ministry. So imo when Harry alerted Snape and Snape alerted the Order, they both had done all they could. It’s kinda pointless to argue that Harry should have gone back to Snape from the Forest, or that Snape should have behave more trustworthy to Harry in Umbridge's office.