If there are screws, there would be some latches around the casing. But some manufacturers use glue on the latches too. If that is the case, u need to force and break the latches with screwdriver. Assuming u are ready to junk the casing and use the HDD in another casing.

Very hard. I have opened Maxtor ones, they use glue on the latches, because they do not want the casing to make squeaking sounds and feels flimsy. Even though i managed to open the case without damaging it, the latches did not survive. But u could either glue it back, or just use some nice tape to hold the casing back again. U could use those carbon fibre look stickers to cover and hold it back again. I doubt they use double sided tape. Its faster and easier to use glue.

There are some techniques to open plastics. Some require soaking it in hot water to make it soft. Some requires to use a few screwdrivers and pry it open very small section at a time. Some spray WD40 or glue remover to make the glue soft before prying it open. Those are what modellers use anyway. But good luck.