Re: "labelling deepfakes with permanent unique metadata or other identifiers"
In most cases, deepfakes of living people will already be illegal under various applicable laws, so I'm guessing anybody creating them isn't going to be too fussed about complying with new laws anyway. Whether it's a watermark or more subtle forms of steganography, these would be relatively simple to remove - especially if you configure a relatively simple AI service to remove it.
I work for a regulator, non-compliant stuff is an endless problem for us, and despite having laws and standards that say what is permitted, plus the power to seize dodgy stuff, we play a continuous game of whack-a-mole against companies who choose not to comply with the rules. As they're invariably outside the UK there's limits to what we can do to track back to the source, and the big tech firms remove content or listings when the authorities complain, but do nothing to stop new ones being added. The EU I believe has similar thoughts to the Indian authorities, I daresay the Anglophone countries will go in a similar direction.
We could lock things down (products, software, services, information) to a point where non-compliance is absolutely minimal, and attempts to evade the regulations more likely to be detected than not, but that would be a very different world to the one most Western citizens live in. I foresee AI regulators will be created, and do things to prove their worth, without being fast enough or comprehensive enough to actually deal with the issues they were created for.