Oops - you are right, maybe your post was not the best one to answer to among the two-three that made similar claims. Anyway, it still applies to what you wrote in point 2: I meant to say that Google here is in a significantly stronger position than "the operators of most networking equipment between Alice/Bob and OWS's servers", because it has access to both endpoints. The attack I have described would not work for Alice's ISP (unless it also happens to be Bob's ISP).
It is significantly easier for Google to match up senders and receivers. Even if you they go through millions of messages per second, in an exchange of, like, 20 IMs they can see if the timestamps of Alice's sent messages pair up almost perfectly with those of Bob's received messages. My ISP cannot do that, unless they see both halves of the conversation.
So, IANACryptographer, but if I understand correctly: Google gets metadata when Alice sends a message (because connect to its server using this "fronting"), and when Bob receives one (because Signal delivers messages using GCM). It doesn't look too hard for them to reconstruct that Alice is exchanging messages to Bob.
This is a circular argument. To quote GP, why would energy be constant? Maybe the variability is beyond our ability to observe. Maybe thermodynamics is wrong, and free energy can be produced but only in very small quantities.