Is it possible to multiplex TLS connections by SNI metadata without knowing any keys?
is it possible with TLS, if the P does not know any of the keys?

Sure. SNI is part of the ClientHello message extensions, which is the first TLS message the client sends. At this point you do not yet have key material on the client side to encrypt this in all cases and even for session resumption, SNI is usually included to tell the server which old key material to use.
Though it may be that this setup breaks in the future if and when encrypted SNI (currently only a draft standard) is enforced.
A (rather minimal) example implementation of such a multiplexer is the dedicated slt package, but this functionality should be achievable with most standard reverse proxy setups, e.g. Apache.