But does it mean that they can easily access and manipulate/write data everywhere? Can they run every process and then simply dump the ram and get the unencrypted data? I know that more work needs to be done, but is this basically game over as far as security goes for the PS4 (at least up to firmware 1.76)?
This is not an easy answer. If you are root, you can do anything or let's better say, you have the rights to do anything. That doesn't mean you can access everything because certain things might be encrypted and are not viewable decrypted in RAM but by now we don't know what's possible. We have to wait until hackers did more fiddling with the system. But they have all the "rights" to do what they want.