My dad said that the PLA wouldn't need to prepare or mobilize much for this operation, because they already secretly stationed troops and assets in Fujian.
Though I still think they'll still need to mobilize their naval assets, but I'm guessing for the first few amphibious landing and blockade, they'll use commercial and civilian fishing boats to quickly establish a beachhead and harass/blockade any foreign ships from intervening.
No. There is no way they can hide that many people from social media/satellites, not even in a military culture as secrecy obsessed as the PLA.