1st link is invalid, it wasn't sold for 28 keys to LTK | Arthor, look at the historic of the item!

1st link: the sale was made 2 months ago and since this trade has changed hands over 6-7 times.. so im not sure this can be used as valid proof.

2nd link: looks good

3rd: as you said it is invalid as this was over 3 months ago and this was the same hat that was sold in your second link, however that was 5 trades later down the track.

4th link: invalid as you said, because although he clicked that he had accepted the offer of the robo glove, the hat never went to him so the trade wasnt made.

5th link: still pretty young

6th link: doesn't really provide much evidence, the guy just might not have wanted to sell it or hadnt found an offer he liked.

So at this stage you only have 1 valid sale, and 1 unsold that is relevant to your suggestion. I am not disagreeing with the price drop but i think you need to provide some more evidence to do so