Does HTTP2 have guarantees that the connection is really reused for multiple requests or could a user agent (or HTTP library) silently use multiple connections for different requests?

I think this makes a major difference for RPC applications, because with a single connection you have a guarantee that all messages will be retrieved in the same order that they were sent.

E.g. with a persistent websocket connection I have the guarantee that all messages will be in order. With mulitple HTTP/1.1 requests I don't have it. Don't know about HTTP/2.

And of course the requirements about such a thing depend on how you design your RPC API (stateless or not).