Certificate validation isn't needed unless the attacker can do a direct MITM

at connection time, which is a lot harder to maintain than injecting a
client.reconnect.

Surely the TCP connection will be reset once the route reconfiguration is completed, either by the MITM server or by the client TCP stack when it discovers the server doesn't know about the connection anymore?

TLS without cert validation defeats the point, you can still be connected to a MITM at any point by anyone who can simply interrupt or corrupt the stream, forcing a reconnect.