Key escrow and other three-way encryption algorithms exist. You wouldn't want to use them in any normal circumstances, but there's no technical reason why you couldn't. A key for you, your recipient, and your friendly government agent.
Very few people actually write cryptographic code, and even fewer write code that is safe to use. I doubt many people will bother with some kind of messenger hook/overlay keyboard to supplant the native messenger encryption. After all, in some countries SMS/MMS is still in use and nobody seems to care about encryption there.
That depends on how easy to use you make this kind of software. An app that makes the soft keyboard produce encrypted output on the fly could indeed become a popular solution until it's prohibited. Like always, the problem is key exchange, though.
Very few people actually write cryptographic code, and even fewer write code that is safe to use. I doubt many people will bother with some kind of messenger hook/overlay keyboard to supplant the native messenger encryption. After all, in some countries SMS/MMS is still in use and nobody seems to care about encryption there.