According to the answer I got from Microsoft Q & A, there is indeed no guarantee that the session B reader would read the writes made by the Session A writer in the correct order, i.e. works like eventual consistency.

Also the session consistency level does not include the consistent prefix guarantee (outside of the session). However assuming you are doing reads via the SDK, you could indeed explicitly request consistent prefix guarantees for each of the read requests you are making as the "session" B reader. Naturally you would lose any session related functionality when doing it.