As an ATCO doing the TMA sectors, I think you are quite right to be frustrated with inaccurate information. Most of the time all we get displayed to us is a bit of paper saying what the delay is. We pass it to you and everything should be ok. It seems nowadays this bit of paper is frequently wrong. We look stupid and it places you in a tricky (or even dangerous) situation. Can I ask that you report the situation through your company (ASR? - I'm not sure what you call it)
As for slowing down a long way out, usually this is not added to your total delay, it is done to save fuel and stop you all screaming into the holds and slowing at the slp's.
There are occasions when we can see that you have already been numbered into the sequence, so we tell you to slow up and you will start absorbing your delay en route but I doubt it is ever more than 30/40 miles out.
When we issue EAT's then these are generally done much further out - perhaps up to 150 miles.
Room for improvement.....................?