Although drinking plenty of fluids helps, to loose weight one must consume less calories than they burn. Also the idea of a temporary restricrive and unsustainable diet is flawed. One must change his/her eating habits permanently and find a balance that works for them.
I had some that was over a year past the expiration date and it tasted fine. Once the best before date has passed, the tea is still safe to consume, however you may notice the flavour may not be as strong as it was, compared to when it was first purchased. This is mainly because the natural oils and flavours in the tea will evaporate over time.
I don't see why you couldn't mix milk with it, but it is a very mild, gentle tea, no harshness whatsoever. Unless you just like milk with your tea, it really isn't necessary.