The cryptography is standard AES-128 in OCB3 mode. It's been around long enough, and has had enough security scrutiny to at least discover a few minor DoS vulnerabilities, that it isn't entirely unreviewed.
For the cipher itself, see https://en.wikipedia.org/wiki/OCB_mode#Attacks
Mosh has been effectively unmaintained since long before QUIC even existed. It should be rewritten to use QUIC, except that QUIC's requirement for TLS certificates rather than generic asymmetric cryptography basically breaks any integrations like this.
And as a side-benefit, if your SSH daemon only listens on the WireGuard interface, that's another layer of defense you get for free (not to mention you'll stop getting noise in your logs).
Ironically though, here you actually need to know about `<Enter>~.` because if the remote host actually goes down, WG will keep trying to contact the remote peer for some time; this is the same behavior that allows you to keep a connection open even when roaming, but seen from the other side.
It’s so annoying that the connection is lost when going to sleep or network issues. And the solutions to fix this are not really worth the effort.